The first confirmed case of a woman contracting HIV from another woman during sex has been reported by the Centres for Disease Control and Prevention in the United States.
Although there have been reports of women transmitting the virus to other women through sexual activity, they have been difficult to prove. For nearly all the cases, other risk factors were present - including sexual contact with men, intravenous drug use, tattooing, piercing or other, potentially risky, behaviour.
This time, the case seems pretty iron-clad.
As outlined in a paper published by the CDC this week, the two women involved were in a monogamous relationship for six months. At the start of their relationship one of the women was HIV-positive, the other was not. The woman who began the relationship HIV negative said it had been 10 years since she had had sex with a man. She did not use intravenous drugs, or get tattoos or acupuncture or transfusions or transplants.
She did, however, occasionally sell her blood plasma, and had tested negative for HIV after donating plasma in March 2012.
Ten days later, she went to the emergency room with symptoms including sore throat, fever, muscle aches, dry cough, vomiting and frequent diarrhoea. She was tested for HIV then, but the results were negative. Two and a half weeks later she went to sell her plasma again. That time she tested positive.
The two women said they had unprotected sex, shared sex toys, and had sex when one of them was menstruating.