SAN FRANCISCO, Feb 1 (AFP) - The human immunodeficiency virus (HIV) can be transmitted through oral sex, contrary to theories that the practice is safe, according to a study published here Tuesday.

"Despite lower transmission risk, oral sex may be an important mode of HIV transmission due to its frequency," the report by researchers at the Center for Disease Control in Atlanta, Georgia and the University of California at San Francisco.

The study, presented at the seventh conference on Retroviruses and Opportunistic Infections, examined 122 men who were infected with HIV between 1996 and 1999, more than six percent of who were infected through oral sex.

"Even with stringent criteria for classification, oral risk transmission represented 6.6 percent of infections in our study of primary HIV," the study said. "Each of the men believed oral sex presented no or minimal risk."

The authors said more study is needed to understand how the virus is transmitted through oral contact.

"The finding has substantial prevention implications," the authors wrote. "Standardized investigation of HIV transmission via oral sex is needed to understand the epidemiology and risk of this behavior."

The only safe sex is abstinence or monogamous intimacy with a healthy partner, according to Dr. Helene Gayle, head of AIDS research at the Centers for Disease Control.

Scientists at the conference Tuesday also heard the results of computer analysis that indicate the AIDS epidemic was traced to a virus that may have come into existence as far back as 1910.

Using a data base of genetic information and supercomputer at Los Alamos National Laboratory in New Mexico, scientist Bette Korber deduced that the virus that causes AIDS mutated from a single ancestor.

At the conference, the original virus was called "HIV Eve."

Korber traced HIV Eve back to about 1930, but said it is possible the virus made its first appearance in people as early as 1910 or as recently as 1950.

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