While such sexually transmitted diseases as gonorrhea and syphilis invade the body with as many as 10, 20, 100 or 200 bacteria, , scientists from the University of Alabama at Birmingham (UAB) have found that most HIV infections are the result of a single copy of the virus penetrating the body’s defenses, The Birmingham News reports (al.com, 5/20).

“In the vast majority of cases a single virus has gone across the sexual mucosa, and that virus has infected a cell,” said George M. Shaw, lead author on the UAB study published online on Monday by the Proceedings of the National Academies of Science. “That cell then makes a lot of virus. Now you just have a firestorm of HIV replication in the next couple [of] weeks. Very quickly the person is populated by millions of viruses.”

In addition to breaking new ground in understanding the HIV transmission process, this study also showed why condoms are so effective in preventing infection and why efforts to develop viable microbicide gels have proven less so. Shaw told The Birmingham News that unlike microbicides studied thus far—which may not stop all individual viruses from entering the body—a properly used condom stops all of them.