The Centers for Disease Control and Prevention (CDC) confirmed this week, in its publication Morbidity and Mortality Weekly Report, that a woman in Texas became infected with HIV through sexual contact with her female partner.

Sexually transmitted HIV between two women has always been considered possible, but is very rare and often hard to prove, as women may have engaged in other behaviors that carry more risk, such as heterosexual sex or sharing needles for intravenous drug use. The woman in this case, who became infected in 2012, had not had heterosexual sex in over ten years, and had only three female partners in the last five years and one partner in the six months prior to infection. In addition, she did not use intravenous drugs, and had not recently gotten a piercing, tattoo, blood transfusion, or organ transplant, which are all known routes of infection. More importantly, the CDC explains that the 46-year-old woman “had a virus virtually identical to that of her female partner, who was diagnosed previously with HIV and who had stopped receiving antiretroviral treatment in 2010.”

HIV is present in bodily fluids such as blood, semen, vaginal secretions, and breast milk. The virus can be transmitted when these fluids come into contact with a sore, a cut, a wound, or mucous membranes in an uninfected person. HIV cannot be transmitted through unbroken skin such as that on a person’s hand. This is why certain sexual activities are riskier than others—unprotected vaginal and anal intercourse are riskiest, for example, because the vagina, anus, and urethra are lined with mucous membranes. The skin around these areas is also more delicate and can tear more easily. But, the risk is not the same for both partners—the receptive partner is at increased risk because semen comes into contact with mucous membranes and any cuts and sores that may be present. Moreover, semen often stays in the vagina or anus for some time, giving it more opportunity to enter the blood stream.

Sexual behaviors between two women, which might include mutual masturbation, oral sex, rubbing genitals together, or penetration with fingers or sex toys, among other things, carry less risk of body fluids coming into contact with mucous membranes or blood. As such, it has always been considered safer, and women who have sex exclusively with other women have been thought of as very low risk for sexually transmitted HIV. While this remains true, this case should remind us that though rare, women-to-women transmission can happen.

The women in this case told CDC investigators that they frequently had sex that was rough enough to cause one or the other of them to bleed. They also shared sex toys back and forth, and had unprotected sex during menstruation. All of these things increased the risk that the HIV-positive woman could transmit the virus to her partner. In addition, the HIV-positive partner had stopped taking her antiretroviral drugs. Had she kept taking them, she likely would have had less virus in her blood and been less likely to transmit it to her partner.

This case should also remind all people, regardless of the sex of their partners, that there are simple steps couples can take to reduce their risk of transmitting HIV. Using condoms is the most important thing that couples engaging in penetrative sex can do to prevent HIV and other sexually transmitted diseases. Condoms can also be used to cover sex toys, and then changed before the toy is used on a different person. This can prevent germs and fluids from being passed back and forth. If a couple doesn’t have a condom, they should wash the sex toy in between uses. Minimizing the chance of blood and cuts—either by being gentler or using lubricant—can also cut down on risk. Finally, in couples where one partner is known to be HIV-positive, continuing antiretroviral treatment can reduce the amount of virus in that person’s blood, making it less likely that he or she transmits HIV.

Though this case should by no means cause a panic, it should remind us all that practicing safer sex is always a good idea.