Читать книгу Basic Virology - Martinez J. Hewlett - Страница 19

The interaction between viruses (and other infectious agents) and their hosts is a dynamic one. As effective physiological responses to infectious disease have evolved in the organism and (more recently) have developed in society through application of biomedical research, viruses themselves respond by exploiting their naturally occurring genetic variation to accumulate and select mutations to become wholly or partially resistant to these responses. In extreme cases, such resistance will lead to periodic or episodic reemergence of a previously controlled disease – the most obvious example of this process is the periodic appearance of human influenza viruses causing disease.

The accelerating rate of human exploitation of the physical environment and the accelerating increase in agricultural populations afford some viruses new opportunities to “break out” and spread both old and novel diseases. Evidence of this is the ongoing acquired immune deficiency syndrome(AIDS) epidemic, as well as sporadic occurrences of viral diseases such as hemorrhagic fevers in Asia, Africa, and the southwestern United States. Investigation of the course of a viral disease, as well as societal responses to it, provides a ready means to study the role of social policies and social behavior of disease in general.

The recent worldwide spread of AIDS is an excellent example of the role played by economic factors and other aspects of human behavior in the origin of a disease. There is strong evidence to support the view that the causative agent, human immunodeficiency virus(HIV), was introduced into the human population by an event fostered by agricultural encroachment of animal habitats in Equatorial Africa. This is an example of how economic need has accentuated risk.

HIV is not an efficient pathogen; it requires direct inoculation of infected blood or body fluids for spread. In the Euro‐American world, the urban concentration of homosexual males with sexual habits favoring a high risk for venereal disease had a major role in spreading HIV and resulting AIDS throughout the male homosexual community. A partial overlap of this population with intravenous drug users and participants in the commercial sex industry resulted in spread of the virus and disease to other portions of urban populations. The result is that in Western Europe and North America, AIDS has been a double‐edged sword threatening two disparate urban populations: the relatively affluent homosexual community and the impoverished heterosexual world of drug abusers – both highly concentrated urban populations. In the latter population, the use of commercial sex as a way of obtaining money resulted in further spread to other heterosexual communities, especially those of young, single men and women.

An additional factor is that the relatively solid medical and financial resources of a large subset of the “economic first world” resulted in wide use of whole blood transfusion and, more significantly, pooled blood fractions for therapeutic use. This led to the sudden appearance of AIDS in hemophiliacs and sporadically in recipients of massive transfusions due to intensive surgery. Luckily, the incidence of disease in these last risk populations has been reduced owing to effective measures for screening blood products.

Different societal factors resulted in a different distribution of HIV and AIDS in Equatorial Africa and Southeast Asia. In these areas of the world, the disease is almost exclusively found in heterosexual populations. This distribution of AIDS occurred because a relatively small concentration of urban commercial sex workers acted as the source of infection of working men living apart from their families. The periodic travel by men to their isolated village homes resulted in the virus being found with increasing frequency in isolated family units. Further spread resulted from infected women leaving brothels and prostitution to return to their villages to take up family life.

Another important factor in the spread of AIDS is technology. HIV could not have spread and posed the threat it now does in the world of a century ago. Generally lower population densities and lower concentrations of individuals at risk at that time would have precluded HIV from gaining a foothold in the population. Slower rates of communication and much more restricted travel and migration would have precluded rapid spread; also, the transmission of blood and blood products as therapeutic tools was unknown a century ago.

Of course, this dynamic interaction between pathogen and host is not confined to viruses; any pathogen exhibits it. The study and characterization of the genetic accommodations that viruses make, both to natural resistance generated in a population of susceptible hosts and to human‐directed efforts at controlling the spread of viral disease, provide much insight into evolutionary processes and population dynamics. Indeed, many of the methodologies developed for the study of interactions between organisms and their environment can be applied to the interaction between pathogen and host.