Viral life cycle

Viruses are similar to other 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 live 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. Generally, viruses are not regarded as living organisms as they are unable to live on their own.

Overview

As stated above, the viral life cycle is dependent on a host cell. A virus is unable to replicate on its own or use "raw" materials on which to survive. Hence a virus will remain dormant until it is able to infect the next host, activate and replicate. Some viruses can live in an open environment for a short time, in some cases, only a few hours. Therefore, viruses use the most efficient method to locate a host, create progeny, and spread to other hosts [N.J. Dimmock et al. "Introduction to Modern Virology, 6th edition." Blackwell Publishing, 2007.] .

Exposure of host

Usually viral infection occurs when a virus enters the host, either:

* through a physical breach (a cut in the skin)
* direct inoculation (mosquito bite [Bureau for Public Health Division of Surveillance and Disease Control. "Take precautions against mosquitos, dead birds to prevent West Nile, encephalitis viruses." W V Med J. 2005 Mar-Apr;101(2):90.] )
* direct infection of the surface itself (inhalation of the virus onto trachea [Quan FS, Compans RW, Nguyen HH, Kang SM. "Induction of Heterosubtypic Immunity to Influenza Virus by Intranasal Immunization." J Virol. 2007 Nov 21] )It is usually only after a virus enters a host that it can gain access to possible susceptible cells.

Viral Entry

In order for the virus to reproduce and thereby establish infection, it must enter cells of the host organism and use those cells' materials. In order to enter the cells, proteins found 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 mechanisms. 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.

Viral shedding

After a virus has made many copies of itself, it usually has exhausted the cell of its resources. The cell is now no longer useful to the virus, therefore it 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

A virus may choose to hide within another cell, either to evade the host cell defenses or immune system, 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 will not produce any progeny, it will remain inactive until external stimuli (such as light or stress) prompts it into activation.

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