01 Jun 11 HIV CAN BE SUBDIVIDED IN SEVERAL WAYS

The most basic subdivision is into the two main types: HIV-1 and HIV-2. The main group within HIV-1 had its formative period of evolution in central Africa, where the potential for sexual transmission has been very high. The corresponding area for HIV-2 was West Africa, where the potential for sexual transmission has been variable but generally lower. Accordingly, HIV-1 destroys the immune system and brings about AIDS more rapidly than HIV-2. Similar geographic patterns occur within each HIV type. HIV-2 infections tend to be more benign in Senegal, where the potential for sexual transmission is relatively low, than in the Ivory Coast and Guinea-Bissau, where the potential for sexual transmission is higher. The subtypes of HIV-1 that occur in Thailand and in east-central Africa, where the potential for sexual transmission has been high, appear to be particularly nasty.

Similar trends hold for the other major human retrovirus: human T-cell lymphotropic virus (HTLV). Infections in Japan, where the potential for transmission is low, tend to cause lethal cancers and paralysis at a lower rate than they do in the Caribbean, where the potential for sexual transmission is high. In Japan most infections tend to be acquired from mothers, through breast milk. The lethal blood cell cancers, which occur in about one out of every twenty-five infected people, tend to arise late in life; about half occur in people who are over sixty years old. Virtually all these cancers appear to arise from infections that were acquired early in life from mothers. The time between infection and the onset of cancer therefore tends to be about sixty years. In the Caribbean about half the people who develop cancer are less than 45 years old, and most infections are sexually transmitted. The time between infection and cancer is therefore shorter in the Caribbean, perhaps much shorter. The difference isn’t just a consequence of living in Japan or the Caribbean. A similar difference is apparent among Japanese Americans and Caribbean Americans, who have presumably acquired their viruses from their respective ancestral lands.

Retroviruses like HIV and HTLV use RNA as their genetic material. Because RNA viruses tend to be more mutation-prone than DNA viruses, one could argue that HIV and HTLV are special cases. It could be argued, for example, that these RNA viruses are able to respond more rapidly than DNA viruses to changes in opportunities for sexual transmission. But similar geographical associations hold for the DNA viruses that have been tested. Genital herpes simplex viruses, for example, have been tested for virulence by inoculating them into inbred lines of mice. As expected from the theory, the herpesviruses from Thailand were more lethal than those from Japan.

The human papillomaviruses (HPVs) are DNA viruses that have both genital forms, which cause cervical cancer, and skin forms, which cause warts. Just a few of the genital forms seem to be responsible for almost all cervical cancer. As expected from evolutionary considerations, women who have more sexual partners are more likely to have the dangerous, cancer-causing genital forms, whereas women who have few sexual partners are more likely to have the mild forms. During the war in the former Yugoslavia, notorious for the use of rape as a weapon, the dangerous genital HPVs spread much more rapidly than the mild genital forms. The more dangerous papillomaviruses appear to be particularly suited for transmission where the potential for sexual transmission is high.

The theory has not been tested within species of bacterial pathogens, but the differences among species show a pattern analogous to that of the sexually transmitted viruses. The most deadly of sexually transmitted bacteria is Treponema pallidum, the cause of syphilis. It depends greatly on a high potential for sexual transmission. The agent of gonorrhea, Neisseria gonorrheae, and the sexually transmitted chlamydia, Chlamydia trachomatis, rarely cause death in infected adults, and their maintenance in populations is not so strongly dependent on a high potential for sexual transmission. This difference in dependence is apparent in the relative success of control measures, a point well illustrated by a program that was conducted from 1990 to 1993 to increase condom use among prostitutes in Fukuoka City, Japan. Condom use increased to four times its original level; C. trachomatis infection inched downward by about a quarter, N. gonorrheae was down by almost half, and syphilis dropped by nearly 95 percent.

This current state of knowledge about sexually transmitted diseases complements the picture generated by studies of acute infectious diseases. It again supports the idea that damage caused by infectious diseases is probably not just an aberration. On the contrary, the evidence indicates that damaging relationships between us and the microbes that feed on us can be maintained indefinitely when high levels of host exploitation are favored by natural selection. For sexually transmitted diseases, natural selection apparently leads to damaging relationships when the potential for sexual transmission is high. For the diseases that fall more neatly into the acute category, natural selection apparently leads to damaging relationships when transmission can occur from immobilized hosts.

This conclusion has subtle but far-reaching implications for the nature of human disease: infection has more potential than the other possible causes of disease to harm hosts perpetually. Over the long run, even moderately common and harmful genetic diseases will fail to sow the seeds of their own perpetuation, unless they provide some compensating benefit. Without a compensating benefit, harmful genetic instructions can be maintained only at a frequency that is set by their generation through mutation: at equilibrium, the loss in the harmful genetic instructions that results from their harmful effects must match the rate at which the instructions are being generated by mutation. If a genetic instruction is even moderately harmful, then it can be maintained in the population only as a very uncommon instruction. Even if a disease reduced reproductive success by as little as one tenth of 1 percent when averaged over the entire population, the genetic instructions responsible for the disease would dwindle over time. This logic provides deep insight into one of the most important questions we face as a society today: What are the gravest infectious threats for the wealthy countries? The answer is the chronic plagues.

*16\225\2*