Recently, the CDC confirmed that females are capable of transmitting HIV to their female sex partners. Female to female transmission of HIV has been difficult to prove, though there have been reports of its occurrence in the past. Most often, this mode of transmission is difficult to substantiate because researchers are unable to rule out other risk factors, such as intravenous drug use or sexual contact with males. However, a recent case study of two women in a monogamous relationship, who did not have a history of intravenous drug use, allowed the researchers to rule out the more common risk factors. The CDC states that though this mode of transmission is rare, everyone should take appropriate preventative measures to guard against HIV.
Read more here.