According to the CDC, transmission of HIV between women who have sex with women (WSW) has been reported rarely and is difficult to ascertain because other risk factors almost are present or cannot be ruled out.

"Although rare, HIV transmission between WSW can occur," it said.

"The potential for HIV transmission by female-to-female sexual contact includes unprotected exposure to vaginal or other body fluids and to blood from menstruation, or to exposure to blood from trauma during rough sex."

The CDC described in the report one case in the Philippines who reported sexual contact exclusively with women and said she did not use injection drugs. However, no source of transmission was confirmed about this case.

Another instance of female-to-female HIV transmission was reported for a 20-year-old woman with no other risk behaviors who said she had a two-year relationship and unprotected intercourse with a female partner known to be HIV-infected. The woman and her partner had identical HIV-1 drug resistance mutations, but no phylogenetic linkage testing was conducted, the CDC added.