HIV has been rampant in Africa from the early '80s. More than 20 years ago, the disease appeared in Nairobi (Kenya), infecting 90% of the city's lower-class prostitutes; but some women practicing this "business" for over 13 years have not developed AIDS. Not having AIDS symptoms is not so shocking, since HIV can stay dormant in the body for many years before it starts to kill.

The surprise is that HIV cannot be detected in these women at all; it apparently cannot stick itself in their cells.

There are also cases of people in other high-risk groups, like homosexuals and spouses of infected hemophiliacs, that do not catch the infection. But these prostitutes, so frequently exposed to the HIV for so long come with the strongest proof yet that people can display a natural immunity to HIV, that removes the danger of AIDS. If the precise reason of that protection can be detected, the humankind could finally get an anti-HIV vaccine.

A mixed Kenyan-Canadian team has monitored a group of 25 Nairobi prostitutes monthly for at least six years, in an intensive survey. Each of the them has had sex with hundreds, perhaps thousands, of HIV-positive clients. And they don't use condoms more often than other prostitutes that got infected with HIV and developed AIDS.

Moreover, they have experienced other sexually transmitted infections, like syphilis and gonorrhea. Lead researcher Dr. Francis Plummer of the University of Manitoba suspects the reason may be found in some proteins called human leukocyte group A antigens. They coat the cells' surface, helping detect foreign invaders like viruses. Preliminary results point to the fact that the HIV-free women have HLAs significantly different from the common types found in people that develop AIDS, but the precise way in which these mutated HLAs can reject HIV infection is still unknown.