The interaction of viruses with their hosts is intimate and the product of a long period of evolution during which viruses coevolved with their hosts. Humans cannot survive without a functioning immune system to protect them from viruses. However, this is the result of the long evolutionary history during which hosts and viruses adapted to one another, because viruses in turn cannot survive without their hosts. The example of rabbit myxoma virus demonstrates that the virulence of a virus diminishes if it kills too large a proportion of its hosts too rapidly. We can even speculate that the exceptional virulence of the influenza virus responsible for the 1918 pandemic might have been made possible because of active warfare ongoing at the time. Very ill and dying soldiers continue to be moved about and the virus could continue to spread, perhaps could even spread more readily, if it incapacitated its hosts. On the other hand, the many examples of ways in which viruses modify the immune and cytokine defenses of the host in order to replicate demonstrate that viruses are capable of evolving more virulent forms if it is to their advantage. The end result is an interplay in which viruses and their hosts exist in an uneasy equilibrium punctuated by the emergence of new viruses or the spread of new epidemics accompanied by changes in the immune system that protect against these viruses.
The virulence of a virus for its host depends in part on the epidemiology of the virus, how it gets from one host to another. The herpesviruses set up a lifelong infection in which they are effectively transferred once per generation. It is in the virus's interest not to incapacitate the host so that the host can pass it on perhaps 20 years later, and her-pesviruses cause minor illness or no illness in most humans. On the other hand, arboviruses must cause a viremia (virus circulating in the blood) high enough to infect an insect taking a blood meal. Because many of these viruses are RNA viruses that do not encode functions to ablate the immune response, rapid and vigorous replication is required to establish the viremia before immunity is established, and this is often harmful to the host because many cells are killed in the process. To take another example, respiratory viruses that are transmitted as aerosols or in respiratory secretions must produce enough virus in the respiratory tract so that respiratory droplets expelled by coughing or sneezing will contain sufficient virus to infect a person nearby. These viruses are transmitted in epidemics that can spread rapidly and that require close contact between individuals, and one infected individual can infect dozens or even hundreds of others in a very short time. Thus these viruses need be transmissible only over a short period. Sexually transmitted viruses have different hurdles to overcome. Because the potential for sexual transmission is usually infrequent and one person interacts with a limited number of others, these viruses need to establish infections that last for long periods of time and that do not incapacitate the infected individual, at least not early in the infection.
The close interplay between viruses and their hosts means that the study of viruses continues to tell us much about the hosts. We now know much more about the adaptive immune system, the cytokine system, and about apop-tosis because of recent studies that started with viruses. Continuing studies on viruses have told us much about the function of regulatory genes and cancers. We are confident that the study of viruses will continue to teach us much about human biology.
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