Strategies To Beat The Host

The immune system is the mortal enemy of viruses - in order to survive, they must be able to reproduce in their hosts and either hide from or overcome the immune system long enough to reproduce and infect other organisms. Many viruses have developed clever ways of avoiding or overcoming the human immune system.

Some viruses, including Herpes and Shingles, lie dormant inside body cells for years, hiding from the immune system until conditions are favorable for infection. Herpes recurs periodically throughout the lifetime of an infected person. Shingles is caused by a dormant version of the chicken pox virus, which can hide for decades, only to re-emerge in later life as an entirely different disease.

Most viruses, however, do not have such tricks to avoid detection. Many, such as the rhinoviruses that cause the common cold, are simply highly infectious and spread rapidly from person to person - the immune system rapidly defeats them, but they reproduce and spread quickly enough that they are not easily contained or wiped out.

The operation of B cells and their antibodies, however, poses a serious

Top: Electron micrograph of viruses without antibodies present. Bottom: When antibody is present, the substructure of the virus is obscured, with the viral particles surrounded by a disabling halo of antibody.

problem for most viruses, because once a host has developed an immunity, they cannot be reinfected. This means that the virus's future reproduction is limited to the population that has not yet been exposed to it. Once it has swept through a population, it faces a troublesome shortage of new hosts. The only way which it can continue to survive is for it to mutate enough that the immune systems of its

previous hosts cannot recognize it. All viruses mutate slowly over time, as errors in the copying and recopying of its DNA collect and are passed on to subsequent generations. But there is one set of viruses - the RNA viruses - that are the true masters of the art, mutating with great frequency. Hepatitis C is of this latter type.

In addition to changing their structure by mutation, many viruses also employ a kind of disguise mechanism. Rather than being released as a naked virus particle, a process that often occurs when the infected cell dies, some viruses depart the living cell by "budding" off. In the budding process, a mature virus particle comes to the surface of the cell and becomes surrounded by a piece of the cell's surface membrane. As it leaves the cell its own surface structure is masked by that of the surface structures of the infected cell. Since the cell surface looks like "self" the virus is invisible to the immune system so long as it retains its cell membrane coat. Such viruses are often highly infective, largely because they are very effective in their ability to evade the immune system. Again, this is a strategy that is used by the Hepatitis C virus.