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Endemic (epidemiology)
 
 
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In epidemiology
Epidemiology

Epidemiology is the study of factors affecting the health and illness of populations, and serves as the foundation and logic of interventions made in the interest of public health and preventive medicine....
, an infection
Infection

An infection is the detrimental colonization of a host organism by a foreign species. In an infection, the infecting organism seeks to utilize the host resources to multiply ....
 is said to be endemic (from Greek
Greek language

Greek is an Indo-European languages native to the southern Balkan peninsula, the language of the Greek people. It forms an independent branch within Indo-European....
 en- in or within + demos people) in a population
Population

File:Population density.pngIn biology, a population is the collection of inter-breeding organisms of a particular species; in sociology, a collection of human beings....
 when that infection is maintained in the population without the need for external inputs. For example, chickenpox
Chickenpox

Chickenpox or chicken pox is a highly contagious illness caused by primary infection with varicella zoster virus . It generally begins with a vesicular skin rash appearing in two or three waves, mainly on the body and head rather than the hands and becoming itchy raw pockmarks, small open sores which heal mostly without scarring....
 is endemic (steady state) in the UK, but malaria
Malaria

Malaria is a Vector -borne infectious disease caused by protozoan parasites. It is widespread in Tropics and subtropical regions, including parts of the Americas, Asia, and Africa....
 is not. Every year, there are a few cases of malaria acquired in the UK, but these do not lead to sustained transmission in the population due to the lack of a suitable vector
Vector (biology)

In epidemiology, a vector is an organism that does not cause disease itself but that transmits infection by conveying pathogens from one Host to another, serving as a transmission ....
 (mosquitoes of the genus Anopheles
Anopheles

Anopheles is a genus of mosquito . There are approximately 460 recognised species: while over 100 can transmit human malaria, only 30-40 commonly transmit parasites of the genus Plasmodium that cause malaria which affects humans in endemic areas....
).

For an infection to be endemic, each person who becomes infected with the disease must pass it on to exactly one other person (on average).






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In epidemiology
Epidemiology

Epidemiology is the study of factors affecting the health and illness of populations, and serves as the foundation and logic of interventions made in the interest of public health and preventive medicine....
, an infection
Infection

An infection is the detrimental colonization of a host organism by a foreign species. In an infection, the infecting organism seeks to utilize the host resources to multiply ....
 is said to be endemic (from Greek
Greek language

Greek is an Indo-European languages native to the southern Balkan peninsula, the language of the Greek people. It forms an independent branch within Indo-European....
 en- in or within + demos people) in a population
Population

File:Population density.pngIn biology, a population is the collection of inter-breeding organisms of a particular species; in sociology, a collection of human beings....
 when that infection is maintained in the population without the need for external inputs. For example, chickenpox
Chickenpox

Chickenpox or chicken pox is a highly contagious illness caused by primary infection with varicella zoster virus . It generally begins with a vesicular skin rash appearing in two or three waves, mainly on the body and head rather than the hands and becoming itchy raw pockmarks, small open sores which heal mostly without scarring....
 is endemic (steady state) in the UK, but malaria
Malaria

Malaria is a Vector -borne infectious disease caused by protozoan parasites. It is widespread in Tropics and subtropical regions, including parts of the Americas, Asia, and Africa....
 is not. Every year, there are a few cases of malaria acquired in the UK, but these do not lead to sustained transmission in the population due to the lack of a suitable vector
Vector (biology)

In epidemiology, a vector is an organism that does not cause disease itself but that transmits infection by conveying pathogens from one Host to another, serving as a transmission ....
 (mosquitoes of the genus Anopheles
Anopheles

Anopheles is a genus of mosquito . There are approximately 460 recognised species: while over 100 can transmit human malaria, only 30-40 commonly transmit parasites of the genus Plasmodium that cause malaria which affects humans in endemic areas....
).

For an infection to be endemic, each person who becomes infected with the disease must pass it on to exactly one other person (on average). Assuming a completely susceptible population, that means that the basic reproduction number
Basic reproduction number

In epidemiology, the basic reproduction number of an infection is the mean number of secondary cases a typical single infected case will cause in a population with no immunity to the disease in the absence of interventions to control the infection....
 (R0) of the infection must equal 1. In a population with some immune
Immune system

An immune system is a collection of biological processes within an organism that protects against disease by identifying and killing pathogens and tumour cells....
 individuals, the basic reproduction number multiplied by the proportion of susceptible individuals in the population (S) must be 1. This takes account of the probability
Probability

Probability, or wikt:chance, is a way of expressing knowledge or belief that an Event will occur or has occurred. In mathematics the concept has been given an exact meaning in probability theory, that is used extensively in such areas of study as mathematics, statistics, finance, gambling, science, and philosophy to draw conclusions about t...
 of each individual to whom the disease may be transmitted
Transmission (medicine)

In medicine, transmission is the passing of a disease from an infected individual or group to a previously uninfected individual or group.The microorganisms that cause disease may be transmitted from one person to another by one or more of the following means:...
 actually being susceptible to it, effectively discounting the immune sector of the population.

For the disease to be in an endemic steady state:

In this way, the infection neither dies out nor does the number of infected people increase exponentially but the infection is said to be in an endemic steady state. An infection that starts as an epidemic
Epidemic

In epidemiology, an infection that is epidemic appears as new cases in a given human population, during a given period, at a rate that substantially exceeds what is "expected," based on recent experience ....
 will eventually either die out (with the possibility of it resurging in a theoretically predictable cyclical manner) or reach the endemic steady state, depending on a number of factors, including the virulence
Force of infection

In epidemiology, force of infection is the rate at which susceptible individuals become infected by an infectious disease. Because it takes account of susceptibility it can be used to compare the transmission risks and rates between different groups of the population for the same infectious disease, or even between different infectious disea...
 of the disease and its mode of transmission
Transmission (medicine)

In medicine, transmission is the passing of a disease from an infected individual or group to a previously uninfected individual or group.The microorganisms that cause disease may be transmitted from one person to another by one or more of the following means:...
.

If a disease is in endemic steady state in a population, the relation above allows us to estimate the R0
Basic reproduction number

In epidemiology, the basic reproduction number of an infection is the mean number of secondary cases a typical single infected case will cause in a population with no immunity to the disease in the absence of interventions to control the infection....
 (an important parameter
Parameter

In mathematics, statistics, and the mathematical sciences, a parameter is a quantity that defines certain characteristics of systems or function s....
) of a particular infection. This in turn can be fed into the mathematical model
Mathematical modelling in epidemiology

It is possible to mathematically model the progress of most infectious diseases to discover the likely outcome of an epidemic or to help manage them by vaccination....
 of an epidemic.

It should be noted that while it might be common to say that AIDS
AIDS

Acquired immune deficiency syndrome or acquired immunodeficiency syndrome is a disease of the human immune system caused by the HIV ....
 is "endemic" in Africa, this is a use of the word in its colloquial form (meaning found in an area). AIDS cases in Africa are still increasing, so the disease is not in an endemic steady state. It is more correct to call the spread of AIDS in Africa an epidemic.

See also


  • Epidemic
    Epidemic

    In epidemiology, an infection that is epidemic appears as new cases in a given human population, during a given period, at a rate that substantially exceeds what is "expected," based on recent experience ....
  • Infectious disease
    Infectious disease

    An infectious disease is a clinically evident disease resulting from the presence of pathogenic microbial agents, including pathogenic viruses, pathogenic bacteria, Mycosis, protozoa, multicellular parasites, and aberrant proteins known as prions....
  • Pandemic
    Pandemic

    A pandemic is an epidemic of infectious disease that spreads through populations across a large region; for instance a continent, or even worldwide....
  • Mathematical modelling in epidemiology
    Mathematical modelling in epidemiology

    It is possible to mathematically model the progress of most infectious diseases to discover the likely outcome of an epidemic or to help manage them by vaccination....
  • Syndemic
    Syndemic

    Syndemic refers to the concentration of two or more diseases or other health conditions in a population in which there is some level of biological interaction among the diseases and health conditions that magnifies the negative health effects of one or more of the co-present diseases or health conditions....