How a virus infects a host is crucial to understanding how infections begin after individual virus particles infiltrate the host's cells. This can be accomplished because viruses are so tiny that they may slip through cellular defenses unopposed. Once inside a host cell, a virus moves to the cell's nucleus where all the DNA and RNA functions as instructions for the cell's operation. By inserting its own genetic information into the cell's nucleus a virus can hijack its function, causing it to produce and release more virus particles so the infection spreads. Another common effect of this cellular hijacking is the damage, breakdown and eventual death of the cell itself.

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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 (e.g.mosquito bite)

•     direct infection of the surface itself (inhalation of the virus into trachea)

It is usually only after a virus enters a host that it can gain access to possible susceptible cells.

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Viral entry

For the virus to reproduce and thereby establish infection, it must enter 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 replication of the viral genome may commence.

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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 by the millions.

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Viral shedding

After a virus has made many copies of itself, it has usually 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.

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Virus latency

Some viruses can "hide" within a 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 does not produce any progeny, it remains inactive until external stimuli—such as light or stress—prompts it to activate.