Using a new statistical method and one of the world's most powerful computers, scientists at the Los Alamos National Laboratory said Tuesday that they had traced the origin of the AIDS epidemic to around 1930, nearly 30 years before the earliest known infection in humans.
The virus could have originated from 1910 to 1950, but 1930 seems the most probable date based on calculations of the AIDS virus' family tree and the rate at which the virus mutated over time, Dr. Bette Korber, the head of the research team, told the Seventh Conference on Retroviruses and Opportunistic Infections.
Scientists generally believe that HIV-1, the AIDS virus, evolved from a virological cousin in chimpanzees that jumped species to humans.
The 1930 date "does not necessarily mark the point where a chimp bit a man," Korber said, because the chimpanzee virus could have been introduced into humans many times earlier without further transmission.
Korber said that the epidemic, which has infected 40-million people worldwide, presumably had its origins in one person or a small group around 1930. But, she added, "there is no way to know" whether the original HIV-1 from 1930 came from a human or chimpanzee or whether the earliest infections caused illness.
An earlier estimate set the origin of HIV-1 in the World War II period, about 15 years later. That estimate was based on the earliest known isolation of HIV-1. It came from blood collected in 1959 from a man in what was then Belgian Congo.
Korber said that her team, which included her husband, James Theiler, a statistical astronomer, had worked for nearly two years to develop a new method to analyze genetic information from the virus DNA of infected people.
Oral sex called perilous
The risk of transmitting the AIDS virus through oral sex is higher than experts previously thought, according to a government-funded study.
About 8 percent of a group of 102 gay and bisexual men acquired HIV through oral-genital contact, researchers found _ four to eight times higher than many scientists believed. Moreover, that's probably an underestimate of the real risk, one federal health official said Tuesday.
About 85 percent of the participants said they did not use condoms for oral sex because they believed that it represented no or minimal risk in transmitting HIV.
Oral sex has a much lower risk of transmitting HIV than intercourse. Some men who substituted only oral sex for anal sex to limit their risk "felt burned" when they learned they still had become infected, said Dr. Frederick Hecht, an author of the study from the Centers for Disease Control and Prevention and the University of California at San Francisco.
Avoiding ejaculation did not eliminate the risk, Hecht said.