THE earliest confirmed case of infection with HIV-1, the virus that causes most AIDS cases around the world, is in a blood sample taken from an African man in 1959, in what was then the Belgian Congo.
Earlier this year, D.N.A. analysts at the Aaron Diamond AIDS Research Center in New York, comparing that sample with others and calculating mutation rates, estimated that the killer virus came into existence sometime in the late 1940's or early 1950's.
If that's the case, what happened? Where did it go? How could the virus, which is now threatening to kill a quarter of some African countries, have gone virtually unnoticed for 20 years -- and then emerged in America, decimating gay men and hemophiliacs in the late 70's before starting on a ruthless march of conquest around the world?
Was it a failure of medical detection? Racist indifference to Africa by the West? Something in the rapidly mutating virus itself?
The short answer is: No one knows for sure. But there are pretty good theories.
The history of the epidemic is a jigsaw puzzle of helpful and unhelpful facts, disproven former "facts" and a lot of notions, both scientific and paranoid. Each new fact sweeps big chunks of the puzzle onto the floor.
Making matters worse, it's probably the most geopolitically touchy disease since porphyria drove England's 18th century kings mad. It makes the bubonic plague look politically correct. Everyone hates rats and fleas, but AIDS can't be discussed without talking about matters at the core of human nature, desire or habit: sex, blood, homosexuality, heroin-shooting, walking skeletons, brain viruses, even monkey-eating.
The most paranoid theory -- that the disease was created in a United States Army germ warfare lab built in Maryland in 1977 -- was exposed as K.G.B. propaganda first published in 1985. This latest D.N.A. analysis, performed last February, demonstrates that the disease long predates the lab.
Many scientists have long thought that the H.I.V. virus came from African monkeys. Several varieties of S.I.V. (simian immunodeficiency virus) have existed harmlessly, probably for thousands of years, in green monkeys, mangabeys and baboons, but not in Asian or South American monkeys. Monkeys are eaten in the central African rain forest, and raw monkey brain is said to be a delicacy in parts of Zaire, just as it is in Hong Kong.
The assumption is that the virus jumped to humans who had eaten or butchered monkeys, or had been bitten by monkeys. That could have happened any time -- or many times -- in the last 10,000 years. It probably happened at least three times, because H.I.V.-2, the weaker strain prevalent in Guinea-Bissau, is closely related to the S.I.V. in local mangabeys. The same is true for H.I.V.-O, a rare strain found in Cameroon. Neither is genetically close to H.I.V.-1, the virulent strain that has been killing people from central Africa to San Francisco to Bangkok.
But H.I.V.-1 didn't necessarily jump to humans as a killer. Some scientists believe a weak early form was caught by European colonists as early as 1900, and one scientist blames it for an outbreak of P.C.P. pneumonia, which is associated with AIDS, in Danzig, Germany, in 1939. Others are skeptical.
Somewhere, probably west of Lake Victoria, and sometime, possibly in the 1940's, H.I.V.-1 mutated inside a human host into an attacker of T-cells and rapid destroyer of immune systems.
It must have spread very slowly at first, in rural areas. It certainly was not killing large numbers of Africans.
"If the disease had been widespread in the 1950's and 1960's, there were enough experienced clinicians in Africa to have noticed something like that going on," said Dr. Anne Buve, an epidemiologist with the Institute for Tropical Medicine in Antwerp, Belgium.
SOME TIME in the 1960's or 1970's, said Dr. Peter Piot, a former researcher in Zaire who is now the executive director of UNAIDS in Geneva, the virus probably moved out of Zaire's rural areas to the cities, and then spread from the continent.
It may have gone to Haiti with the French-speaking Haitians who filled up Zaire's civil service after the Belgians fled in 1960. It may have gone straight to America, perhaps with the Peace Corps. It may have gone to Cuba with soldiers returning from northern Angola.
In any case, it ultimately came to public attention because it somehow got into a population vastly different from central Africa's. The gay communities in San Francisco and New York were small and insular. Anal sex and promiscuity sped the virus through them. Gay Americans were also well educated, well served by doctors and well organized in a civil rights struggle. When mysterious deaths began to hit, they noticed immediately and shouted loudly. Within five years of the late 70's, when fears focused on rumors of "gay cancer," the disease had a name and its virus had been found. Books and TV movies followed.
Meanwhile, in Africa, AIDS was making steady progress, but with less public attention. As early as 1981, doctors in Paris and Brussels diagnosed it among wealthy Zaireans treated in Europe. By 1983, doctors from the Institute for Tropical Medicine and the Centers for Disease Control were hunting it in Africa.
As Mirko Grmek's "History of AIDS" (Princeton University Press, 1990) describes a 1985 conference on African AIDS, only a few cases had been confirmed in Africa. "Belgian and French physicians attending the meeting insisted a true epidemic was under way," Mr. Grmek wrote. "The representatives of affected countries took offense and insisted on denying the problem."
Another quirk: most of the first hard-hit countries were Francophone, to which the British and American press pay less attention.
Slowly, more facts turned up. A Danish surgeon who died in 1977, apparently of AIDS, had worked in rural Zaire from 1972 to 1975. An increase in Kaposi's sarcoma dating to mid-century was recalled. More aggressive Kaposi's hit Zambia and Uganda in 1982.
WHEN researchers traveled through remote western Uganda in 1985 to study "slim disease," which appeared in the early 1980's, they brought new AIDS-test kits, and all 29 "slim" victims proved H.I.V.-positive. A 1987 study by Dr. Piot of 454 blood samples frozen after a 1976 Ebola virus outbreak in Zaire found five H.I.V.-positive ones.
AIDS migrated out along truck routes, hitting hardest in border towns where truckers wait for days to clear customs. It was also still mutating; -- there are now 10 subgroups of H.I.V.-1, all fatal.
Now a new question is stumping scientists: Why is AIDS hitting eastern and southern Africa so much harder than it did the area where it originated? Among pregnant women visiting clinics in Kinshasa, the capital of Congo, and nearby Yaounde, Cameroon, infection rates reached a plateau that was below 10 percent. But in eastern and southern African cities from Kampala, Uganda, to Richard's Bay, South Africa, they have hit 30 percent and are still rising.
No one knows why. The virus may have mutated. Southern Africans may have less resistant genes. There may be something in sexual habits, venereal disease rates, circumcision practices.
Figuring out exactly where it all started, some scientists say, is less important. "People have become pretty pragmatic," Dr. Buve said. "They are asking, 'Do we need to know? Let's rather look at the future.' ".