The Washington Post 
   
   By Daniel Q. Haney 
   AP Medical Editor
   Monday, February 1, 1999; 1:15 a.m. EST
   
   CHICAGO (AP) -- Mystery solved: The AIDS virus came from chimps.
   
   Experts have wondered about the origin of HIV ever since the epidemic
   emerged almost two decades ago. The uncertainty launched a variety of
   conspiracy theories, some suggesting that AIDS was a government plot,
   created purposely to kill.
   
   Now, research presented at an AIDS conference Sunday provides what
   scientists say is convincing proof to the contrary: The virus got its
   start in the forests of Africa when humans caught it from chimpanzees.
   In fact, they say the virus has spread at least three times from
   chimps to people.
   
   ``This is absolutely evidence to put (conspiracy theories) to rest,''
   Dr. Constance Benson of the University of Colorado said.
   
   Even scientists who scorned those theories have been unsure where AIDS
   actually arose. Some suspected chimps, while others thought monkeys or
   other primates could have been the source.
   
   The latest discovery was made by Dr. Beatrice Hahn of the University
   of Alabama at Birmingham, who tracked HIV's ancestor to a virus that
   has long infected one of the four subspecies of chimp that live in
   Africa.
   
   She said, ``We conclude that this subspecies is the source of the
   human AIDS virus,'' which now infects about 35 million people
   worldwide.
   
   Experts believe that HIV -- the virus that causes AIDS -- is a recent
   affliction of people. At last year's Conference on Retroviruses and
   Opportunistic Infections, Dr. David Ho and others from the Aaron
   Diamond AIDS Research Center at Rockefeller University presented
   evidence that the virus probably first infected humans in the 1940s or
   early '50s.
   
   At the opening of this year's meeting, Hahn made the case that this
   event almost certainly occurred in west equatorial Africa. It could
   have happened when someone was bitten by a chimp, or a hunter was
   exposed to contaminated blood while field dressing an animal.
   
   Hahn said her team nailed down the connection by analyzing frozen
   tissue saved from a chimp named Marilyn that died from complications
   of childbirth at a U.S. Air Force primate center in New Mexico 14
   years ago.
   
   The chimp version of the AIDS virus is called SIVcpz. It is extremely
   rare among chimps in U.S. labs, perhaps because these animals are
   removed from the wilds as babies and so are never exposed to the virus
   sexually. Until recently, it had been isolated only three times.
   
   Hahn's discovery of the fourth began when a colleague cleaning out a
   lab freezer ran across Marilyn's specimens and sent them to her. The
   researchers performed various kinds of genetic analyses that were
   unavailable at the time the chimp died.
   
   Marilyn's tissue was found to harbor SIVcpz. The Alabama team used
   molecular analysis techniques to study Marilyn's virus plus the other
   three examples discovered earlier.
   
   They found that three of the four are about as genetically similar to
   the human AIDS virus as they are to each other. They include one gene,
   called vpu, that is also part of HIV but not the other AIDS-like
   viruses that infect monkeys.
   
   All three samples were found to have come from Pan troglodytes
   troglodytes, which is one of the four subspecies of chimp in Africa.
   These animals lives in Cameroon, Equatorial Guinea, Congo and Central
   African Republic, the region where AIDS is thought to have started.
   
   The fourth sample was much less like HIV, and it came from another
   subspecies of chimp that is native to East Africa.
   
   There are three major groups of HIV in people, code-named M, N and O.
   M is the variety that has spread around the world, while N and O are
   seen only in west-central Africa.
   
   The natural habitat of Pan troglodytes troglodytes perfectly overlaps
   the area where these three groups were first recognized. The
   researchers believe that each group arose from a separate
   chimp-to-human transmission of SIVcpz.
   
   Hahn said a French team, headed by Dr. Phillippe Mauclere of the
   Pasteur Institute, recently found three more chimps infected with
   SIVcpz at a game sanctuary in Cameroon. One sample has been
   genetically analyzed, and it too closely resembles HIV.
   
   ``That nails it,'' said Hahn.
   
   Chimps, which have probably carried the virus for hundreds of
   thousands of years, apparently do not get sick from it. Researchers
   say that figuring out why could offer clues for helping people fight
   HIV.
   
   Hahn's work is being published in this week's issue of the journal
   Nature.
   
                    Copyright 1999 The Associated Press