HIV-1 is the virus that causes most cases of AIDS around the world. Another form of the virus, HIV-2, is less easily transmitted.

"The majority of HIV-1 infected individuals worldwide are women who acquire HIV infection following sexual contact. Blocking HIV transmission and local spread in the female lower genital tract is key to prevent infection and ultimately to ease the pandemic," wrote study authors Dr. Florian Hladik and Dr. M. Juliana McElrath, of the Fred Hutchinson Cancer Research Center in Seattle.

Using a unique model system they developed, the scientists found that HIV-1 simultaneously enters two different types of cells in the vaginal epithelium (outer lining of vaginal cells) associated with the immune system -- Langerhans cells and CD4+ T-cells.

Both these cells can migrate out of the vaginal epithelium.

"Our findings provide exciting, definitive insights into the initial events of HIV-1 infection in the human vagina, which can guide the design of effective strategies to block local transmission and prevent HIV-1 spread," McElrath said.