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An infection is the colonization of a host
Host (biology)
In biology, a host is an organism that harbors a parasite, or a mutual or commensal symbiont, typically providing nourishment and shelter. In botany, a host plant is one that supplies food resources and substrate for certain insects or other fauna...

 organism by parasite
Parasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host. Traditionally parasite referred to organisms with lifestages that needed more than one host . These are now called macroparasites...

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease
A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...

. Colloquially, infections are usually considered to be caused by microscopic organisms or microparasite
A microparasite is a parasite that complete a full life cycle within one host and can be transmitted directly to conspecific hosts. They often reproduce within a host's cells and are generally too small to be seen with the naked eye. Most are viruses, bacteria and fungi with a smaller number being...

s like virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

es, prion
A prion is an infectious agent composed of protein in a misfolded form. This is in contrast to all other known infectious agents which must contain nucleic acids . The word prion, coined in 1982 by Stanley B. Prusiner, is a portmanteau derived from the words protein and infection...

s, bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

, and viroid
Viroids are plant pathogens that consist of a short stretch of highly complementary, circular, single-stranded RNA without the protein coat that is typical for viruses. The smallest discovered is a 220 nucleobase scRNA associated with the rice yellow mottle sobemovirus...

s, though larger organisms like macroparasite
Macroparasites are parasites that are large enough to be seen with the naked eye, in contrast to microparasites. They grow in one host but reproduce by infective stages outside of this host. These generally include ticks, mites, nematodes, flatworms, etc., and can be either external parasites or...

s and fungi
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...

 can also infect.

Hosts normally fight infections themselves via their immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...

. Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

ian hosts react to infections with an innate
Innate immune system
The innate immune system, also known as non-specific immune system and secondary line of defence, comprises the cells and mechanisms that defend the host from infection by other organisms in a non-specific manner...

 response, often involving inflammation
Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...

, followed by an adaptive
Adaptive immune system
The adaptive immune system is composed of highly specialized, systemic cells and processes that eliminate or prevent pathogenic growth. Thought to have arisen in the first jawed vertebrates, the adaptive or "specific" immune system is activated by the “non-specific” and evolutionarily older innate...

 response. Pharmaceuticals can also help fight infections.

The branch of medicine
Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....

 that focuses on infections and pathogens is infectious disease
Infectious disease
Infectious diseases, also known as communicable diseases, contagious diseases or transmissible diseases comprise clinically evident illness resulting from the infection, presence and growth of pathogenic biological agents in an individual host organism...


Diagnostic approach

Diagnosis is the identification of the nature and cause of anything. Diagnosis is used in many different disciplines with variations in the use of logics, analytics, and experience to determine the cause and effect relationships...

 of infections can be difficult as specific signs and symptoms are rare. If an infection is suspected, blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

, urine
Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...

 and sputum
Sputum is mucus that is coughed up from the lower airways. It is usually used for microbiological investigations of respiratory infections....

 cultures are usually the first step. Chest x-rays and stool
Human feces
Human feces , also known as a stool, is the waste product of the human digestive system including bacteria. It varies significantly in appearance, according to the state of the digestive system, diet and general health....

 analysis may also aid diagnosis. Spinal fluid
Cerebrospinal fluid
Cerebrospinal fluid , Liquor cerebrospinalis, is a clear, colorless, bodily fluid, that occupies the subarachnoid space and the ventricular system around and inside the brain and spinal cord...

 can be tested to ensure that there is no brain infection.

In children the presence of cyanosis
Cyanosis is the appearance of a blue or purple coloration of the skin or mucous membranes due to the tissues near the skin surface being low on oxygen. The onset of cyanosis is 2.5 g/dL of deoxyhemoglobin. The bluish color is more readily apparent in those with high hemoglobin counts than it is...

, rapid breathing, poor peripheral perfusion, or a petechial rash increases the risk of a serious infection by greater than 5 fold. Other important indicators include parental concern, clinical instinct, and temperature greater than 40 °C.

Signs and symptoms

  • Extreme fatigue which may be ongoing for more than 2–3 months
  • Continued weight loss
  • Low grade or spiking fever
  • Night sweats and chills
  • Vague body aches and pain

Bacterial or viral

Bacterial and viral infections can both cause symptoms such as malaise
Malaise is a feeling of general discomfort or uneasiness, of being "out of sorts", often the first indication of an infection or other disease. Malaise is often defined in medicinal research as a "general feeling of being unwell"...

, fever
Fever is a common medical sign characterized by an elevation of temperature above the normal range of due to an increase in the body temperature regulatory set-point. This increase in set-point triggers increased muscle tone and shivering.As a person's temperature increases, there is, in...

, and chills. It can be difficult to distinguish which is the cause of a specific infection. It's important to distinguish, because viral infections cannot be cured by antibiotics.
Comparison of viral and bacterial infection
Characteristic Viral infection Bacterial infection
Typical symptoms In general, viral infections are systemic. This means they involve many different parts of the body or more than one body system at the same time; i.e. a runny nose, sinus congestion, cough, body aches etc. They can be local at times as in viral conjunctivitis or "pink eye" and herpes. Only a few viral infections are painful, like herpes. The pain of viral infections is often described as itchy or burning. The classic symptoms of a bacterial infection are localized redness, heat, swelling and pain. One of the hallmarks of a bacterial infection is local pain, pain that is in a specific part of the body. For example, if a cut occurs and it is infected with bacteria, pain will occur at the site of the infection. Bacterial throat pain is often characterized by more pain on one side of the throat. An ear infection is more likely to be diagnosed as bacterial if the pain occurs in only one ear. A possibly infected cut that produces pus and milky-colored liquid is most likely infected.
Cause Pathogenic viruses  Pathogenic bacteria
Pathogenic bacteria
Pathogenic bacteria are bacteria that cause bacterial infection. This article deals with human pathogenic bacteria.Although the vast majority of bacteria are harmless or beneficial, quite a few bacteria are pathogenic...


There is a general chain of events that applies to infections. For infections to occur a given chain of events must occur. The chain of events involves several steps which include the infectious agent, reservoir, entering a susceptible host, exit and transmission to new hosts. Each of the links must be present in a chronological order for an infection to develop. Understanding these steps helps health care workers target the infection and prevent it from occurring in the first place.


Infection begins when an organism successfully colonizes by entering the body, growing and multiplying. Most humans are not easily infected. Those who are weak, sick, malnourished, have cancer or are diabetic have increased susceptibility to chronic or persistent infections. Individuals who have a suppressed immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...

 are particularly susceptible to opportunistic infections. Entrance to the host generally occurs through the mucosa
Mucous membrane
The mucous membranes are linings of mostly endodermal origin, covered in epithelium, which are involved in absorption and secretion. They line cavities that are exposed to the external environment and internal organs...

 in orifices like the oral cavity, nose, eyes, genitalia, anus, or open wounds. While a few organisms can grow at the initial site of entry, many migrate and cause systemic infection in different organs. Some pathogens grow within the host cells (intracellular) whereas others grow freely in bodily fluids.

A wound is a type of injury in which skin is torn, cut or punctured , or where blunt force trauma causes a contusion . In pathology, it specifically refers to a sharp injury which damages the dermis of the skin.-Open:...

 colonization refers to nonreplicating microorganisms within the wound, while in infected wounds replicating organisms exist and tissue is injured. All multicellular organism
Multicellular organism
Multicellular organisms are organisms that consist of more than one cell, in contrast to single-celled organisms. Most life that can be seen with the the naked eye is multicellular, as are all animals and land plants.-Evolutionary history:Multicellularity has evolved independently dozens of times...

s are colonized to some degree by extrinsic organisms, and the vast majority of these exist in either a mutualistic or commensal relationship with the host. An example of the former would be the anaerobic bacteria
Anaerobic organism
An anaerobic organism or anaerobe is any organism that does not require oxygen for growth. It could possibly react negatively and may even die if oxygen is present...

 species which colonize the mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

ian colon
Colon (anatomy)
The colon is the last part of the digestive system in most vertebrates; it extracts water and salt from solid wastes before they are eliminated from the body, and is the site in which flora-aided fermentation of unabsorbed material occurs. Unlike the small intestine, the colon does not play a...

, and an example of the latter would be the various species of staphylococcus
Staphylococcus is a genus of Gram-positive bacteria. Under the microscope they appear round , and form in grape-like clusters....

 which exist on human skin
Human skin
The human skin is the outer covering of the body. In humans, it is the largest organ of the integumentary system. The skin has multiple layers of ectodermal tissue and guards the underlying muscles, bones, ligaments and internal organs. Human skin is similar to that of most other mammals,...

. Neither of these colonizations would be considered infections. The difference between an infection and a colonization is often only a matter of circumstance. Organisms which are non-pathogenic can become pathogenic given specific conditions, and even the most virulent organism requires certain circumstances to cause a compromising infection. Some colonizing bacteria, such as Corynebacteria sp. and viridans streptococci, prevent the adhesion and colonization of pathogenic bacteria and thus have a symbiotic relationship with the host, preventing infection and speeding wound healing
Wound healing
Wound healing, or cicatrisation, is an intricate process in which the skin repairs itself after injury. In normal skin, the epidermis and dermis exists in a steady-state equilibrium, forming a protective barrier against the external environment...


The variables involved in the outcome of a host becoming inoculated by a pathogen and the ultimate outcome include:
  • the route of entry of the pathogen and the access to host regions that it gains
  • the intrinsic virulence
    Virulence is by MeSH definition the degree of pathogenicity within a group or species of parasites as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host. The pathogenicity of an organism - its ability to cause disease - is determined by its...

     of the particular organism
  • the quantity or load of the initial inoculant
  • the immune
    Immune system
    An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...

     status of the host being colonized

As an example, the staphylococcus
Staphylococcus is a genus of Gram-positive bacteria. Under the microscope they appear round , and form in grape-like clusters....

 species present on skin remain harmless on the skin, but, when present in a normally sterile space, such as in the capsule of a joint
A joint is the location at which two or more bones make contact. They are constructed to allow movement and provide mechanical support, and are classified structurally and functionally.-Classification:...

 or the peritoneum
The peritoneum is the serous membrane that forms the lining of the abdominal cavity or the coelom — it covers most of the intra-abdominal organs — in amniotes and some invertebrates...

, will multiply without resistance and create a huge burden on the host.


A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...

 can arise if the host's protective immune mechanisms are compromised and the organism inflicts damage on the host. Microrganisms can cause tissue damage by releasing a variety of toxins or destructive enzymes. For example, Clostridium tetani
Clostridium tetani
Clostridium tetani is a rod-shaped, anaerobic bacterium of the genus Clostridium. Like other Clostridium species, it is Gram-positive, and its appearance on a gram stain resembles tennis rackets or drumsticks. C. tetani is found as spores in soil or in the gastrointestinal tract of animals. C...

 releases a toxin which can paralyze muscles, or staphylococcus
Staphylococcus is a genus of Gram-positive bacteria. Under the microscope they appear round , and form in grape-like clusters....

 releases toxins which can produce shock and sepsis. Not all infectious agents cause disease in all hosts. For example less than 5% of individuals infected with polio develop disease. On the other hand, some infectious agents are highly virulent. The prion
A prion is an infectious agent composed of protein in a misfolded form. This is in contrast to all other known infectious agents which must contain nucleic acids . The word prion, coined in 1982 by Stanley B. Prusiner, is a portmanteau derived from the words protein and infection...

 causing mad cow disease
Bovine spongiform encephalopathy
Bovine spongiform encephalopathy , commonly known as mad-cow disease, is a fatal neurodegenerative disease in cattle that causes a spongy degeneration in the brain and spinal cord. BSE has a long incubation period, about 30 months to 8 years, usually affecting adult cattle at a peak age onset of...

 and Creutzfeldt–Jakob disease kills almost all animals and people that are infected.

Persistent infections occur because the body is unable to clear the organism after the initial infection. Persistent infections are characterized by the continual presence of the infectious organism often as latent infection with occasional recurrent relapses of active infection. There are some viruses that can maintain a persistent infection by infecting different cells of the body. Some viruses once acquired never leave the body. A typical example is the herpes virus which tends to hide in nerves and become reactivated when specific circumstances arise. Persistent infections cause millions of deaths globally each year. Chronic infections by parasites account for a high morbidity and mortality in many underdeveloped countries.

Primary and secondary infections

Primary and secondary infection may either refer to succeeding infections or different stages of one and the same infection such as in acute herpes labialis infection.
In the latter case, acute infection may also be used, as in acute HIV infection
Acute HIV infection
Acute HIV infection or primary HIV infection is the second stage of HIV infection. It occurs after the incubation stage, before the latency stage and the potential AIDS succeeding the latency stage....



In order for infecting organisms to survive and repeat the cycle of infection in other hosts, they (or their progeny) must leave an existing reservoir and cause infection elsewhere. Transmission of infections can take place via many potential routes. Infectious organisms may be transmitted either by direct or indirect contact. Direct contact occurs when an individual comes into contact with the reservoir. This may mean touching infected bodily fluids or drinking contaminated water or being bitten by the deer tick
Deer Tick
Deer Tick is an American indie folk band from Providence, Rhode Island led by guitarist and singer-songwriter John McCauley. Deer Tick's music has been described as a combination of folk, blues, and country.-History:...

. Direct contact infections can also result from inhalation of infectious organisms found in aerosol particles emitted by sneezing or coughing. Another common means of direct contact transmission involves sexual activity - oral, vaginal, or anal sex.

Indirect contact occurs when the organism is able to withstand the harsh environment outside the host for long periods of time and still remain infective when specific opportunity arises. Inanimate objects that are frequently contaminated include toys, furniture, door knobs, tissue wipes or personal care products from an infected individual. Consuming food products and fluid which have been contaminated by contact with an infecting organism is another case of disease transmission by indirect contact.

A common method of transmission in under developed countries is fecal-oral transmission. In such cases, sewage water is used to wash food or is consumed. This results in food poisoning. Common pathogens which are transmitted by the fecal-oral route include Vibrio cholerae
Vibrio cholerae
Vibrio cholerae is a Gram-negative, comma-shaped bacterium. Some strains of V. cholerae cause the disease cholera. V. cholerae is facultatively anaerobic and has a flagella at one cell pole. V...

, Giardia
Giardia is a genus of anaerobic flagellated protozoan parasites of the phylum Metamonada in the supergroup "Excavata" that colonise and reproduce in the small intestines of several vertebrates, causing giardiasis, commonly known as Beaver fever...

species, rotavirus
Rotavirus is the most common cause of severe diarrhoea among infants and young children, and is one of several viruses that cause infections often called stomach flu, despite having no relation to influenza. It is a genus of double-stranded RNA virus in the family Reoviridae. By the age of five,...

es, Entameba histolytica, Escherichia coli
Escherichia coli
Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...

, and tape worms. Most of these pathogens cause gastroenteritis
Gastroenteritis is marked by severe inflammation of the gastrointestinal tract involving both the stomach and small intestine resulting in acute diarrhea and vomiting. It can be transferred by contact with contaminated food and water...


All the above modes are examples of horizontal transmission
Horizontal transmission
Horizontal transmission is the transmission of a bacterial, fungal, or viral infection between members of the same species that are not in a parent-child relationship....

 because the infecting organism is transmitted from person to person in the same generation. There are also a variety of infections transmitted vertically
Vertical transmission
Vertical transmission, also known as mother-to-child transmission, is the transmission of an infection or other disease from mother to child immediately before and after birth during the perinatal period. A pathogen's transmissibility refers to its capacity for vertical transmission...

 - that is from mother to child during the birthing process or fetal development. Common disorders transmitted this way include AIDs, hepatitis, herpes, and cytomegalovirus

Specific bacterial infections

H pylori
Helicobacter pylori
Helicobacter pylori , previously named Campylobacter pyloridis, is a Gram-negative, microaerophilic bacterium found in the stomach. It was identified in 1982 by Barry Marshall and Robin Warren, who found that it was present in patients with chronic gastritis and gastric ulcers, conditions that were...

 is associated with inflammation of the stomach and is a common cause of stomach ulcers
Peptic ulcer
A peptic ulcer, also known as PUD or peptic ulcer disease, is the most common ulcer of an area of the gastrointestinal tract that is usually acidic and thus extremely painful. It is defined as mucosal erosions equal to or greater than 0.5 cm...

 and gastritis
Gastritis is an inflammation of the lining of the stomach, and has many possible causes. The main acute causes are excessive alcohol consumption or prolonged use of nonsteroidal anti-inflammatory drugs such as aspirin or ibuprofen. Sometimes gastritis develops after major surgery, traumatic...

. At least 10 percent of individuals infected with h pylori develop an ulcer. Moreover, there is an increased risk of stomach cancer
Stomach cancer
Gastric cancer, commonly referred to as stomach cancer, can develop in any part of the stomach and may spread throughout the stomach and to other organs; particularly the esophagus, lungs, lymph nodes, and the liver...

 after an infection with this organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...


Methicillin-resistant staphylococcus aureus
Methicillin-resistant Staphylococcus aureus
Methicillin-resistant Staphylococcus aureus is a bacterium responsible for several difficult-to-treat infections in humans. It is also called multidrug-resistant Staphylococcus aureus and oxacillin-resistant Staphylococcus aureus...

 predominantly affects the skin and is considered to be a super bug as it is very resistant to antibiotics. This bacteria is known to generate a variety of toxic enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

s which can lead to vomiting, diarrhea
Diarrhea , also spelled diarrhoea, is the condition of having three or more loose or liquid bowel movements per day. It is a common cause of death in developing countries and the second most common cause of infant deaths worldwide. The loss of fluids through diarrhea can cause dehydration and...

, shock and sepsis. MRSA is quite common in hospitals and today there is a great cause for concern about its spread.

Chronic ear infections

Chronic ear infections
Otitis media
Otitis media is inflammation of the middle ear, or a middle ear infection.It occurs in the area between the tympanic membrane and the inner ear, including a duct known as the eustachian tube. It is one of the two categories of ear inflammation that can underlie what is commonly called an earache,...

 are a common problem in childhood. These infections may be due to bacteria or the common cold virus. The disorder often presents with persistent blockage of the ear, hearing loss, chronic ear drainage, balance problems, deep ear pain, headache, fever, excess sleepiness or confusion. Chronic ear infections usually develop slowly over many years in patients who have had ear problems. The treatment of persistent ear infections is complex and requires a combination of antibiotics, corticosteroids, and/or placement of tubes. When this fails surgery is required to control the infection.


Osteomyelitis simply means an infection of the bone or bone marrow...

 is a bone infection caused by various bacteria and can occur in both children and adults. When bone gets infected, there is continuous pain, fever and it is painful to move the extremity. Bone infections are acquired from an infection elsewhere in the body, trauma or spread from adjacent infected tissues. The diagnosis of bone infection requires a bone scan, blood cultures and x rays. Sometimes the bone marrow is aspirated to discover the specific organism. Osteomyelitis is a serious infection and carries a high complication rate if not treated promptly. If the infection is diagnosed rapidly, the prognosis is good. However chronic Osteomyelitis can take years to heal and can keep on recurring. Individuals at risk for Osteomyelitis include those who have artificial joints or metal components in the joint.

Lyme disease

Lyme is a tick borne disease that can cause a skin rash, fever, chills, body aches, and joint pain. Some individuals develop severe weakness and temporary paralysis. Lyme disease is caused by at least three species of bacteria belonging to the genus Borrelia, which are carried by deer ticks. One is more likely to get the infection during the summer months, especially if one spends time in grassy woodlands where ticks breed. When the infection is diagnosed promptly, most people do recover fully. However, there are some individuals who keep on having recurring or lingering symptoms long after the infection has been treated. When it becomes chronic, Lyme Disease can present with a variety of symptoms including migrating joint pains, headaches, confusion, excess fatigue, inability to sleep, paralysis of one side of the face, and difficulty concentrating. Even though there are reliable tests available they are not one hundred percent sensitive. While most individuals do respond to a 14 day course of antibiotics, some individuals take considerably longer.


Chlamydia (bacterium)
Chlamydia is a genus of bacteria that are obligate intracellular parasites. Chlamydia infections are the most common bacterial sexually transmitted infections in humans and are the leading cause of infectious blindness worldwide....

 is a common sexually transmitted disease which can damage the female reproductive organs and result in permanent infertility if not treated promptly. Chlamydia is the most frequently contracted sexually transmitted bacterial infection in America. The organism is transmitted orally and through anal sex. Chlamydia is known as the silent disorder because many women who acquire this infection have no symptoms. Those who develop symptoms may complain of low back pain, painful intercourse, nausea, fever, bleeding, lower abdominal cramps, and a discharge. Chlamydia can be treated with antibiotics; however, women whose sex partners are not treated are at a very high risk for re-infection. Moreover, multiple infections increase the female's chances of sustaining damage to her reproductive organs, resulting in infertility.

Persistent viral infections

Many individuals develop a variety of infections in their lifetime, but quickly overcome them. However, some individuals develop chronic
Chronic (medicine)
A chronic disease is a disease or other human health condition that is persistent or long-lasting in nature. The term chronic is usually applied when the course of the disease lasts for more than three months. Common chronic diseases include asthma, cancer, diabetes and HIV/AIDS.In medicine, the...

 or persistent infections. In the majority of cases, persistent infections are caused by viruses and not bacteria. The common viruses that can cause chronic infection include: measles, hepatitis
Hepatitis is a medical condition defined by the inflammation of the liver and characterized by the presence of inflammatory cells in the tissue of the organ. The name is from the Greek hepar , the root being hepat- , meaning liver, and suffix -itis, meaning "inflammation"...

, herpes, infectious mononucleosis
Infectious mononucleosis
Infectious mononucleosis is an infectious, widespread viral...

 and Cytomegalovirus
Cytomegalovirus is a viral genus of the viral group known as Herpesviridae or herpesviruses. It is typically abbreviated as CMV: The species that infects humans is commonly known as human CMV or human herpesvirus-5 , and is the most studied of all cytomegaloviruses...

 (CMV). Bacteria can also cause chronic infections in individuals with diabetes
Diabetes mellitus
Diabetes mellitus, often simply referred to as diabetes, is a group of metabolic diseases in which a person has high blood sugar, either because the body does not produce enough insulin, or because cells do not respond to the insulin that is produced...

, those with compromised immunity, and in individuals who smoke
Tobacco smoking
Tobacco smoking is the practice where tobacco is burned and the resulting smoke is inhaled. The practice may have begun as early as 5000–3000 BCE. Tobacco was introduced to Eurasia in the late 16th century where it followed common trade routes...


The most common persistent infections in North America include: HIV
Human immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...

, hepatitis, herpes simplex, and, common to all mammals, endogenous retroviruses, which play a crucial role in placentation and therefore act as symbionts. Hepatitis B and C are usually acquired from the use of dirty needles, blood transfusions, or sexual intercourse. HIV has similar modes of transmission. Once hepatitis has been acquired, it becomes a chronic disorder. While some individuals with hepatitis B may remain asymptomatic, many will show active symptoms and remain infectious. In the long term, both hepatitis B and C can cause liver failure or liver cancer
Liver cancer
Liver tumors or hepatic tumors are tumors or growths on or in the liver . Several distinct types of tumors can develop in the liver because the liver is made up of various cell types. These growths can be benign or malignant...

. In some cases, the signs and symptoms of liver damage may not appear for 20 years after the infection was initially acquired. Though not all viral, other persistent infections include: recurrent ear infection in children, tuberculosis, Lyme disease, Chlamydia, and malaria. The problem with recurrent infections is that the organism continues to damage the body which eventually results in symptoms. As the body weakens, the individual develops weight loss and extreme fatigue.

Occult infection

An occult or asymptomatic infection is medical terminology for a "hidden" infection, that is, one which presents no symptoms verifiable and recognizable by a doctor. Dr. Fran Giampietro discovered this type, and coined the term "occult infection" in the late 1930s.

Treatment and prevention

Viable treatment and prevention strategies will disrupt the infection cycle. For example, direct transmission can be diminished by adequate hygiene
Hygiene refers to the set of practices perceived by a community to be associated with the preservation of health and healthy living. While in modern medical sciences there is a set of standards of hygiene recommended for different situations, what is considered hygienic or not can vary between...

, maintaining a sanitary environment, and health education.

When infection attacks the body, anti-infective drugs can suppress the infection. Four types of anti-infective or drugs exist: antibacterial (antibiotic
An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...

), antiviral, antitubercular, and antifungal. Depending on the severity and the type of infection, the antibiotic may be given by mouth, injection or may be applied topical
In medicine, a topical medication is applied to body surfaces such as the skin or mucous membranes such as the vagina, anus, throat, eyes and ears.Many topical medications are epicutaneous, meaning that they are applied directly to the skin...

ly. Severe infections of the brain
Human brain
The human brain has the same general structure as the brains of other mammals, but is over three times larger than the brain of a typical mammal with an equivalent body size. Estimates for the number of neurons in the human brain range from 80 to 120 billion...

 are usually treated with intravenous
Intravenous therapy
Intravenous therapy or IV therapy is the infusion of liquid substances directly into a vein. The word intravenous simply means "within a vein". Therapies administered intravenously are often called specialty pharmaceuticals...

 antibiotics. Sometimes, multiple antibiotics are used to decrease the risk of resistance
Antibiotic resistance
Antibiotic resistance is a type of drug resistance where a microorganism is able to survive exposure to an antibiotic. While a spontaneous or induced genetic mutation in bacteria may confer resistance to antimicrobial drugs, genes that confer resistance can be transferred between bacteria in a...

 and increase efficacy. Antibiotics only work for bacteria and do not affect viruses. Antibiotics work by slowing down the multiplication of bacteria or killing the bacteria. The most common classes of antibiotics used in medicine include penicillin
Penicillin is a group of antibiotics derived from Penicillium fungi. They include penicillin G, procaine penicillin, benzathine penicillin, and penicillin V....

, cephalosporin
The cephalosporins are a class of β-lactam antibiotics originally derived from Acremonium, which was previously known as "Cephalosporium".Together with cephamycins they constitute a subgroup of β-lactam antibiotics called cephems.-Medical use:...

s, aminoglycoside
An aminoglycoside is a molecule or a portion of a molecule composed of amino-modifiedsugars.Several aminoglycosides function as antibiotics that are effective against certain types of bacteria...

s, macrolide
The macrolides are a group of drugs whose activity stems from the presence of a macrolide ring, a large macrocyclic lactone ring to which one or more deoxy sugars, usually cladinose and desosamine, may be attached. The lactone rings are usually 14-, 15-, or 16-membered...

s, quinolone
The quinolones are a family of synthetic broad-spectrum antibiotics. The term quinolone refers to potent synthetic chemotherapeutic antibacterials....

s and tetracyclines.

Techniques like hand washing, wearing gowns, and wearing face masks can help prevent infections from being passed from the surgeon to the patient or vice versa. Frequent hand washing
Hand washing
Hand washing for hand hygiene is the act of cleaning the hands with or without the use of water or another liquid, or with the use of soap, for the purpose of removing soil, dirt, and/or microorganisms....

 remains the most important defense against the spread of unwanted organisms. Nutrition has to be improved and one has to make changes in life style- such as avoiding the use of illicit drugs, using a condom, and entering an exercise program. Cooking foods well and avoiding eating foods which have been left outside for a long time is also important. Do not take antibiotics for longer than needed. Long term use of antibiotics leads to resistance and chances of developing opportunistic infection
Opportunistic infection
An opportunistic infection is an infection caused by pathogens, particularly opportunistic pathogens—those that take advantage of certain situations—such as bacterial, viral, fungal or protozoan infections that usually do not cause disease in a healthy host, one with a healthy immune system...

s like clostridium difficile
Clostridium difficile
Clostridium difficile , also known as "CDF/cdf", or "C...

 colitis. Vaccination
Vaccination is the administration of antigenic material to stimulate the immune system of an individual to develop adaptive immunity to a disease. Vaccines can prevent or ameliorate the effects of infection by many pathogens...

 is another means of preventing infections by facilitating the development of immune resistance in vaccinated hosts.

Fossil record

Evidence of infection in fossil remains is a subject of interest for paleopathologists, scientists who study occurrences of injuries and illness in extinct life forms. Signs of infection have been discovered in the bones of carnivorous dinosaurs. When present, however, these infections seem to tend to be confined to only small regions of the body. A skull attributed to the early carnivorous dinosaur Herrerasaurus ischigualastensis exhibits pit-like wounds surrounded by swollen and porous bone. The unusual texture of the bone around the wounds suggests they were afflicted by a short-lived, non-lethal infection. Scientists who studied the skull speculated that the bite marks were received in a fight with another Herrerasaurus. Other carnivorous dinosaurs with documented evidence of infection include Acrocanthosaurus
Acrocanthosaurus is a genus of theropod dinosaur that existed in what is now North America during the Aptian and early Albian stages of the Early Cretaceous. Like most dinosaur genera, Acrocanthosaurus contains only a single species, A. atokensis. Its fossil remains are found mainly in the U.S...

, Allosaurus
Allosaurus is a genus of large theropod dinosaur that lived 155 to 150 million years ago during the late Jurassic period . The name Allosaurus means "different lizard". It is derived from the Greek /allos and /sauros...

, Tyrannosaurus
Tyrannosaurus meaning "tyrant," and sauros meaning "lizard") is a genus of coelurosaurian theropod dinosaur. The species Tyrannosaurus rex , commonly abbreviated to T. rex, is a fixture in popular culture. It lived throughout what is now western North America, with a much wider range than other...

and a tyrannosaur from the Kirtland Formation
Kirtland Formation
The Kirtland Formation is a sedimentary geological formation. It is the product of alluvial muds and overbank sand deposits from the many channels draining the coastal plain that existed on the inland seashore of North America, in the late Cretaceous period. It overlies the Fruitland Formation...

. The infections from both tyrannosaurs were received by being bitten during a fight, like the Herrerasaurus specimen.

See also

  • Antiseptic
    Antiseptics are antimicrobial substances that are applied to living tissue/skin to reduce the possibility of infection, sepsis, or putrefaction...

  • Autoinfection
    Autoinfection is the infection of a primary host with a parasite, particularly a helminth, in such a way that the complete life cycle of the parasite happens in a single organism, without the involvement of another host. Therefore, the primary host is at the same time the secondary host of the...

  • Coinfection
    In parasitology, coinfection is the term used to describe the simultaneous infection of a host by multiple pathogen species. In virology, coinfection can also refer to the simultaneous infection of a single cell by two or more virus particles...

  • Hand infection
  • Infection control
    Infection control
    Infection control is the discipline concerned with preventing nosocomial or healthcare-associated infection, a practical sub-discipline of epidemiology. It is an essential, though often under-recognized and under-supported, part of the infrastructure of health care...

  • Infectious disease
    Infectious disease
    Infectious diseases, also known as communicable diseases, contagious diseases or transmissible diseases comprise clinically evident illness resulting from the infection, presence and growth of pathogenic biological agents in an individual host organism...

  • List of infectious diseases
  • Multiplicity of infection
    Multiplicity of infection
    The multiplicity of infection or MOI is the ratio of infectious agents to infection targets . For example, when referring to a group of cells inoculated with infectious virus particles, the multiplicity of infection or MOI is the ratio of the number of infectious virus particles to the number of...

  • Opportunistic infection
    Opportunistic infection
    An opportunistic infection is an infection caused by pathogens, particularly opportunistic pathogens—those that take advantage of certain situations—such as bacterial, viral, fungal or protozoan infections that usually do not cause disease in a healthy host, one with a healthy immune system...

  • Routes of infection
  • Ubi pus, ibi evacua
    Ubi pus, ibi evacua
    Ubi pus, ibi evacua is a Latin aphorism or adage, often cited in medicine, meaning "where [there is] pus, there evacuate [it]". It refers to what clinicians should do when there is a collection of pus in the body; that is, to create an opening for it to evacuate...

     (Latin: "where there is pus
    Pus is a viscous exudate, typically whitish-yellow, yellow, or yellow-brown, formed at the site of inflammatory during infection. An accumulation of pus in an enclosed tissue space is known as an abscess, whereas a visible collection of pus within or beneath the epidermis is known as a pustule or...

    , there evacuate it")

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