AIDS will surpass the Black Death as the world?s worst pandemic if the 40 million people living with HIV or AIDS do not get life-prolonging drugs. The illness has killed 25 million people since the early 1980s, and an estimated 14,000 people are infected each day with HIV, especially in Sub-Saharan Africa, home to more than 28 million HIV/AIDS sufferers. Ninety-five percent of new infections are in the world?s poorest countries, where life-prolonging drugs are not available to most people. Without antiretroviral drugs, most people living with HIV/AIDS will die, pushing the death toll beyond the 40 million killed by the Black Death, caused by flea bites, that almost depopulated Asia and Europe in the 14th century.

?Despite the impressive advances in medicine since then, HIV/AIDS is likely to surpass the Black Death as the worst pandemic ever,? says Peter Lamptey, of the Family Health International AIDS Institute. ?If we don?t improve access to treatment in the next 10-15 years, we could have as many as 65 million deaths from this disease.?

In the New York Times Science section for January 29, Gina Kolata reports that AIDS researchers Dr. Beatrice Hahn and Dr. George Shaw, of the University of Alabama, have discovered that many African animals, including big cats such as lions, live long lives with the animal version of AIDS. Their bodies have somehow learned to defeat the deadly consequences of the virus.

Their research began with an event that took place 10 years ago, when a cat owner in California noticed AIDS-like symptoms in her house cats. When she took them to Dr. Niels Pedersen, an animal virologist at the University of California, he diagnosed a close relation to the human AIDS virus. He began to wonder if wild cats were infected as well, especially the big cats of Africa, since it has been assumed that AIDS was initially passed to humans from African primates such as chimpanzees.

Dr. Stephen O?Brien, a cat expert at the National Cancer Institute, began looking for the virus in serum that had been taken from thousands of wild cats, including cheetahs, lions, ocelots and pumas. He was surprised to find that the majority of the wild cats were carrying the virus. ?Every cat was infected with a virus that had the potential to kill the immune system,? O?Brien says. But for some reason, the animals weren?t sick. ?We spent a lot of time looking for a disease, but no one could find it,? he says. Somehow, wild cats had learned to live with the virus and not get sick, but domestic cats, who were new victims, were defenseless against it.

Other researchers discovered the same pattern in primates. Monkeys in the wild carried the virus but were not infected, while laboratory monkeys who received the virus died. ?African primates all carry their own little viruses,? says Dr. Jonathan S. Allan, a virologist at the Southwest Research Foundation in San Antonio, Texas. ?In some species, the viruses have been there for thousands of years. And the natural host never gets sick.?

Researchers Shaw and Hahn compared wild chimpanzees with chimps kept in labs, to see which ones carried the AIDS virus. They expected that most chimpanzees, no matter where they lived, would be infected, but they found only one primate with AIDS?a wild chimp that was healthy. ?It?s quite incredible,? says Dr. Edward Holmes of Oxford University.

This may be because infected chimp populations in the wild are too isolated to pass on the virus to other groups. Or populations infected with the virus may have all succumbed to it and become extinct. Maybe there aren?t as many chimps carrying the virus as we thought. Now scientists want to know: if the virus is so scarce in primates, how did it jump to humans?

Other researchers are looking at these new discoveries and wondering what it means with regard to human AIDS. Why are wild animals that carry the virus so immune to its effects? They believed at first that these animals? immune systems must be destroying the virus. But further tests showed that wasn?t true, since these healthy animals had huge numbers of viruses in their blood.

Then scientists thought that somehow AIDS wasn?t killing off cells. But that was also wrong?viruses were constantly killing off enormous numbers of cells. What was different was, these animals replaced them. Despite having enormous numbers of viruses in their blood, constantly killing off white blood cells, these animals are able to tolerate them.

O?Brien feels this is an example of evolution. ?When a virus gets into a population, like HIV jumped into humans, it can kill off the species or not,? he says. ?If it does not, either the virus becomes weakened or the species changes.?

Dr. Mark Feinburg, of Emory University, believes that with so many people infected in Africa, future generations will have a disproportionate number of people with genetic predispositions to live with the virus, just as many wild animals do now, which is how evolutionary changes occur. ?It?s going to happen,? Feinburg says. ?The severity of the epidemic in some parts of the world is so profound that it will clearly impact human evolution. In the past, we?ve been left to infer what the impact of infection was on human evolution. [Now,] we will have the opportunity to observe it.?

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