Scientists have made a key discovery in pinpointing how HIV is transmitted in men who have sex with men.

The researchers hope that by better understanding how HIV spreads during human sex, that they'll be better able to stop the epidemic.

"By knowing the origin of the transmitted virus, scientists may be able to develop new vaccines, vaginal microbicides and drugs to prevent the spread of sexually transmitted HIV," study author Dr. Davey Smith, an associate professor of medicine at the University of California San Diego, said in a release.

Semen is made up of sperm, white blood cells and the fluid around these cells, known as seminal plasma.

HIV in semen comes in two contagious forms — DNA in the white blood cells and free-floating RNA in the seminal fluid.

Until now, researchers did not know whether HIV RNA or DNA was transmitted during sex.

Smith and his colleagues used genetic analysis to trace the ancestral history of the virus in six pairs of men, the source partners, who sexually transmitted their HIV to other men, the recipient partners.

To investigate, the researchers compared the virus found in the recipients to the DNA and RNA versions in the men who infected them.

RNA was the closest match, leading the team to conclude HIV originated from RNA in the seminal fluid of the source partners.

The findings are reported in this week's issue of Science Translational Medicine.

Davey's team only looked at males, so the findings may not apply to HIV transmission in women.

The results also need to be tested in larger samples of pairs to determine which features of viral genomes are linked with transmission.