I would like to speculate on a slightly different version of this idea. The function of' the viruses may actually be to carry temporarily needed genetic material from one group of organisms to another with the purpose of preparing them for oncoming environmental distress. In other words, the vi-ruses may be the information carriers of some kind of a genetic communication network between living Organisms, including humans.
Chemical communication is a commonly used method in nature. For example, evidence indicates that trees whose leaves are torn to simulate damages done by insects use airborne chemical messengers to induce their undamaged neighbors to increase the concentrations of phenolic compounds in their tissues. These compounds apparently make the leaves less palatable in preparation for a spreading insect population. Viruses may simply be more sophisticated messengers that not only warn their hosts but also bring them the necessary genetic information to strengthen their defenses against an environmental threat.
The immune response that the viruses may eventually evoke in their hosts may be one way the hosts get rid of the information and the messengers that are not needed any more. it is also possible that the milder viral diseases, such as the common cold, are the manifestations of some yet unknown temporary adaptation process induced by the viral genetic material. Since most humans do not live in natural settings as they once did, however, the survival value of a viral infection may not be relevant to the functioning of our bodies anymore. On the other hand, as Thomas suggests, the more serious viral diseases, like influenza or rabies, may be bio logical accidents caused when humans intercept viral messages not intended for them.
In this context, it is interesting to note that certain animals, for example skunks and opossums, are more resistant to rabies infection than are humans.