In response to Nickrz, I was told by a Red Cross nurse that the only way to have “safe” mutual masturbation was with latex gloves on.
In response to handy and The Ryan, lesbians can get HIV through a number of different mechanisms. Bisexuals of both genders can be carriers, so can lesbians who are needle users. All it takes is a few promiscuous lesbians/female bisexuals, interacting with infected individuals and the lesbian communitity at large, to infect a lot of lesbians. The number of individuals who were infected originally may have been small, but promiscous unprotected sex accelerates the spread.