WASHINGTON (Reuters) - The AIDS virus invaded the United States in about 1969 from Haiti, carried most likely by a single infected immigrant who set the stage for it to sweep the world in a tragic epidemic, scientists said on Monday.

Michael Worobey, a University of Arizona evolutionary biologist, said the 1969 U.S. entry date is earlier than some experts had believed.

The timeline laid out in the study led by Worobey indicates that HIV infections were occurring in the United States for roughly 12 years before AIDS was first recognized by scientists as a disease in 1981. Many people had died by that point.

"It is somehow chilling to know it was probably circulating for so long under our noses," Worobey said in a telephone interview.

The researchers conducted a genetic analysis of stored blood samples from early AIDS patients to determine when the human immunodeficiency virus first entered the United States.

They found that HIV was brought to Haiti by an infected person from central Africa in about 1966, which matches earlier estimates, and then came to the United States in about 1969.

The researchers think an unknown single infected Haitian immigrant arrived in a large city like Miami or New York, and the virus circulated for years -- first in the U.S. population and then to other nations.

It can take several years after infection for a person to develop AIDS, a disease that ravages the immune system.

DISEASE MULTIPLIES

"That one infection would have become two, and then it doubles again and the two becomes four," Worobey said. "So you have a period -- probably a fair number of years -- where you're dealing with probably fewer than a hundred people who are infected.

"And then, as with epidemic expansion, at some point the hundred becomes 200, you start getting into thousands, tens of thousands. And then quite rapidly you can be up into the hundreds of thousands of infections that were probably already there before AIDS was recognized in the early 1980s."

The study was published in the journal Proceedings of the National Academy of Sciences.

The path the virus traveled as it jumped from nation to nation has long been debated by scientists.

The University of Miami's Dr. Arthur Pitchenik, a co-author of the study, had seen Haitian immigrants in Miami as early as 1979 with a mystery illness that turned out to be AIDS. He knew the government long had stored some of their blood samples.

The researchers analyzed samples from five of these Haitian immigrants dating from 1982 and 1983. They also looked at genetic data from 117 more early AIDS patients from around the world.

This genetic analysis allowed the scientists to calibrate the molecular clock of the strain of HIV that has spread most widely, and calculated when it arrived first in Haiti from Africa and then in the United States.

The researchers virtually ruled out the possibility that HIV had come directly to the United States from Africa, setting a 99.8 percent probability that Haiti was the steppingstone.

"I think that it gives us more clear insight into the history of it (the AIDS epidemic) and what path the virus took -- and hard objective evidence, not just armchair thinking," Pitchenik said in a telephone interview.

Studies suggest the virus first entered the human population in about 1930 in central Africa, probably when people slaughtered infected chimpanzees for meat. AIDS has killed more than 25 million people and about 40 million others are infected with HIV.