Researchers report that they've discovered the origin of strains of the AIDS virus that affect gay and bisexual men.

"By knowing the origin of the transmitted virus, scientists may be able to expand new vaccines, vaginal microbicides and drugs to avoid the spread of sexually transmitted HIV," study author Dr. Davey Smith, an associate lecturer of medicine at the University of California San Diego.

At subject is the HIV in semen, which is made up of seminal cells and liquid called seminal plasma. HIV particles with RNA inside exist in the fluid, while the decisive cells hold the DNA of HIV, the study authors explained.

The researchers used a type of heritable analysis to study men who had sexually transmitted their HIV to other men.

"Until now, it had not been recognized whether HIV RNA or DNA is transmitted during sex," said Smith. "By analyzing the genetic differences between these two forms and the virus that was eventually transmitted to newly infected individuals; we found that it was the HIV RNA form present in seminal plasma that was transmitted."

What about HIV transmission to women? "Since the vast popular of women are infected with HIV by exposure to the virus in semen, HIV RNA in the seminal plasma is the possible culprit, but this needs to be confirmed.