A genetic analysis of 25-year-old blood samples has outlined a new map of the AIDS virus' journey out of Africa, showing that today's most widespread subtype first emerged in Haiti in the 1960s and arrived in the United States a few years later.

The analysis fills in a gap in the history of the virus, whose migration has been known in only a sketchy form from its origin in Africa in the 1930s to its first detection in Los Angeles in 1981.

Michael Gottlieb, an assistant clinical professor of medicine at University of California, Los Angeles, and one of the original discoverers of AIDS, said the analysis placed the virus in the United States nearly a decade earlier than previously believed.

"It's pretty clear evidence for Haiti as a stepping-stone," he said. "The suggestion that the infection was further below our radar than I'd previously suspected is kind of unnerving."

The analysis, published today in the Proceedings of the National Academy of Sciences, focused on a variety of HIV known as subtype B, which is the most prevalent form in most countries outside of Africa.

Michael Worobey, an evolutionary biologist at the University of Arizona and senior author of the study, analyzed five blood samples collected in 1982 and 1983 from Haitian AIDS patients in Miami. The samples had been stored in a freezer by the national Centers for Disease Control and Prevention.

Worobey and colleagues looked at two viral genes and compared their sequences with virus samples from around the world.

As a baseline, they used virus samples from Central Africa that are considered some of the earliest forms of HIV.

Because viruses are constantly mutating, the researchers could construct a rough timeline of development by measuring how much the genes in later samples had drifted away from their ancestral forms.

The team found that the Haitian samples were genetically the most closely related to the African virus, indicating that they were the earliest to branch off.

Statistically, the researchers found a 99.7 percent certainty that HIV subtype B originated in Haiti as opposed to elsewhere, Worobey said.

He surmised that the virus was brought to Haiti by workers who had gone to Democratic Republic of the Congo, formerly known as Zaire, after the country became independent in 1960.

The virus appears to have been carried to the United States by Haitian immigrants sometime between 1966 and 1972, according to the mutation timeline.

Researchers have debated for years whether the U.S. epidemic came directly from Africa or through Haiti.

They have also debated whether people from the United States exported the virus to Haiti via a sex-tourism trade that flourished in the late 1970s and early 1980s. Worobey said the latest study did "a good job of settling the debate. . . . This shows quite clearly that the data are really only consistent with a Haiti-first origin."

Beatrice Hahn, a virologist at the University of Alabama at Birmingham, who was not involved in the study, said, "I think these calculations are as good as the currently available methods allow."

She cautioned against blaming Haitians or Central Africans for spreading the AIDS epidemic.

"These viruses are fairly clever, and they have to survive," she said. "They will find niches. . . . You realize chance events play a very important role."