A virus is an infectious agent with two goals: to infect and to proliferate. Its life cycle is relatively simple, yet modern medicine is struggling in its fight against different viruses. In most cases the most effective anti-viral treatment is still lots of rest and plenty of water, or in other words – letting the immune system take care of things. The following video portrays the life cycle of a common influenza (flu) virus. While the video describes the mechanisms of the flu virus, it generally pertains to all types of viruses.

This video was produced for ZIRUS by Xvivo.

Disclaimer: the video describes an anti-viral drug development technology. The article should not be regarded as advice to use any of the products of Zirus or any other company for that matter.

Each virus has its own target cell. Some viruses, like the flu virus, infect cells of the respiratory tract, while others, such as the various hepatitis viruses, infect liver cells, and the human immunodeficiency virus (HIV) infects white blood cells. Each virus contains a receptor, a surface protein which mediates its invasion of the cell. It is this receptor that dictates the identity of the cell to be infected and as a result also the symptoms of the ensuing disease. After the virus penetrates into the cell it commandeers various cellular apparatuses for its own needs, that is to replicate and give rise to multiple copies of the virus. Different viruses employ different techniques, but the basic principle is the same – exploiting the cell's protein synthesis and DNA/RNA replication machineries in aim to construct viral structures that are similar to the original virus. The replication rate changes from virus to virus: some viruses work at a slow pace, keeping a "low profile" in order to persist for a long time in the host without being detected by the immune system, while other viruses are much more aggressive, replicating intensively until the infected cell bursts and multitudes of virus particles are spread and can now infect neighboring cells. Some viruses incorporate their genome into the host cell's DNA and remain dormant for a long time, and then break out when the host's immune system weakens, for example at times of psychological stress.

A virus' simplicity and its parasitic nature are highly advantageous when it comes to evading the host immune system. When outside the cell, the virus is exposed to cells of the immune system; however, once it penetrates its target cell a virus becomes much more difficult (but not impossible) for the immune system to recognize it. Detection of an infected cell leads to its destruction by types of white blood cells called natural killer (NK) cells and cytotoxic T lymphocytes (CTLs). The simplicity of some viruses, such as HIV, is manifested by their rather inefficient replication machinery which leads to the generation of many mutations in the viral genome. This leads to a large percentage of the offspring virus particles being dysfunctional, but it also enables the emergence of anti-viral drug resistance. For this reason viral diseases are often treated with drug cocktails – the combination of several drugs which prevents the virus from developing resistance.

Erez Garty
Deparment of Biological Chemistry
Weizmann Institute of Science
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