Introduction

The evolution of virus diseases is distinct from, although related to, the evolution of viruses in general. The emergence of a virus disease can reflect the ‘evolution’ of the causal agent, but a disease can emerge in the absence of any change in the agent. To maintain this distinction we will use a special vocabulary and refer to the ‘emergence, epidemicity, and control’ of virus diseases (Nathanson, 1990).

Propagated infections

In some instances, a new virus disease is the direct consequence of the appearance of an agent which is truly new to the population affected. Recent striking instances are the appearance of canine parvovirus and of human immunodeficiency virus (HIV), both of which were signalled by a global pandemic of disease. It appears that canine parvovirus represents an authentic instance of virus evolution, in which a pre-existing agent, feline panleukopenia virus, underwent a few key point mutations which made it very infectious and pathogenic for the canine population (Parish, 1990). These mutations presumably led to the emergence of canine parvovirus disease. Likewise, it seems probable that HIV evolved from a viral ancestor, such as simian immunodeficiency virus (SIV), which was circulating in subhuman primates. It is likely that this highly plastic agent has undergone some evolution since originally infecting humans, which has enhanced its ability to replicate in human CD4-positive mononuclear cells. A mass of circumstantial data strongly suggests that the virus first invaded the human population in Africa, and that it has spread worldwide only in the last 15 years (Desrosiers, 1990; Getchell et al, 1987; Karpas, 1990; Levy et al, 1986).