Viral life cycle
Encyclopedia
Viruses
are similar to living organisms, however there are differences. One of the ways a virus can be seen as living is that a virus needs to replicate
and create progeny
. However, unlike other organisms, a virus cannot survive on its own. It is only active when replicating within a host, using a hosts' resources and food. Once inside a host, a virus's sole purpose is to make as many copies of itself, and infect other host cells; everything it does is to benefit its fitness
and increase the number of its offspring
.
. . Most viruses are species specific, and they only infect a narrow range of plants, animals, bacteria or fungi.
It is usually only after a virus enters a host that it can gain access to possible susceptible cells.
of the host organism and use those cells' materials. To enter the cells, proteins on the surface of the virus interact with proteins of the cell. Attachment, or adsorption, occurs between the viral particle and the host cell membrane. A hole forms in the cell membrane, then the virus particle or its genetic contents are released into the host cell, where viral reproduction may commence.
. It is at this stage a distinction between susceptibility and permissibility
of a host cell is made. Permissibility determines the outcome of the infection. After control is established and the environment is set for the virus to begin making copies of itself, replication occurs quickly.
, or simply because it is not in the best interest of the virus to continually replicate. This hiding is deemed latency. During this time, the virus does not produce any progeny, it remains inactive until external stimuli
—such as light or stress—prompts it to activate.
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...
are similar to living organisms, however there are differences. One of the ways a virus can be seen as living is that a virus needs to replicate
Replicate
Replicate may refer to:* In biology, replication is a process by which genetic material, a cell, or an organism reproduces or makes an exact copy or copies...
and create progeny
Offspring
In biology, offspring is the product of reproduction, of a new organism produced by one or more parents.Collective offspring may be known as a brood or progeny in a more general way...
. However, unlike other organisms, a virus cannot survive on its own. It is only active when replicating within a host, using a hosts' resources and food. Once inside a host, a virus's sole purpose is to make as many copies of itself, and infect other host cells; everything it does is to benefit its fitness
Fitness (biology)
Fitness is a central idea in evolutionary theory. It can be defined either with respect to a genotype or to a phenotype in a given environment...
and increase the number of its offspring
Offspring
In biology, offspring is the product of reproduction, of a new organism produced by one or more parents.Collective offspring may be known as a brood or progeny in a more general way...
.
Overview
A virus is totally dependent on a host cellCell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
. . Most viruses are species specific, and they only infect a narrow range of plants, animals, bacteria or fungi.
Exposure of host
Usually viral infection occurs when a virus enters the host, either:- through a physical breach (a cut in the skinSkin-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...
) - direct inoculationInoculationInoculation is the placement of something that will grow or reproduce, and is most commonly used in respect of the introduction of a serum, vaccine, or antigenic substance into the body of a human or animal, especially to produce or boost immunity to a specific disease...
(e.g.mosquitoMosquitoMosquitoes are members of a family of nematocerid flies: the Culicidae . The word Mosquito is from the Spanish and Portuguese for little fly...
bite) - direct infectionInfectionAn infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...
of the surface itself (inhalation of the virus into tracheaVertebrate tracheaIn tetrapod anatomy the trachea, or windpipe, is a tube that connects the pharynx or larynx to the lungs, allowing the passage of air. It is lined with pseudostratified ciliated columnar epithelium cells with goblet cells that produce mucus...
)
It is usually only after a virus enters a host that it can gain access to possible susceptible cells.
Viral Entry
For the virus to reproduce and thereby establish infection, it must enter cellsCell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
of the host organism and use those cells' materials. To enter the cells, proteins on the surface of the virus interact with proteins of the cell. Attachment, or adsorption, occurs between the viral particle and the host cell membrane. A hole forms in the cell membrane, then the virus particle or its genetic contents are released into the host cell, where viral reproduction may commence.
Viral replication
Next, a virus must take control of the host cell's replication mechanismsDNA replication
DNA replication is a biological process that occurs in all living organisms and copies their DNA; it is the basis for biological inheritance. The process starts with one double-stranded DNA molecule and produces two identical copies of the molecule...
. It is at this stage a distinction between susceptibility and permissibility
Permissive
When a cell or host is defined as permissive in virology, it refers to the fact that the virus is able to circumvent host defenses and is able to replicate. Usually this occurs when the virus has modulated one or several of the host cellular intrinsic defenses, and the host immune system...
of a host cell is made. Permissibility determines the outcome of the infection. After control is established and the environment is set for the virus to begin making copies of itself, replication occurs quickly.
Viral shedding
After a virus has made many copies of itself, it usually has exhausted the cell of its resources. The host cell is now no longer useful to the virus, therefore the cell often dies and the newly produced viruses must find a new host. The process by which virus progeny are released to find new hosts, is called shedding. This is the final stage in the viral life cycle.Viral latency
Some viruses can "hide" within a cell, either to evade the host cell defenses or immune systemImmune 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...
, or simply because it is not in the best interest of the virus to continually replicate. This hiding is deemed latency. During this time, the virus does not produce any progeny, it remains inactive until external stimuli
Stimulus (physiology)
In physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity....
—such as light or stress—prompts it to activate.