A little known sexually transmitted infection can boost the survival of patients infected with HIV—a more dangerous virus, researchers say. GB virus C (GBV-C) is the only known case of a potentially beneficial STI in humans. But it’s an example of a phenomenon that scientists are beginning to see elsewhere: STIs that are good for your health. What’s more, the health benefits of these helpful STIs could have given a boost to the evolution of promiscuity, scientists say. “There is a common perception that STIs are harmful,” says Chad Smith, an evolutionary ecologist at The University of Texas at Austin. But in a survey of the scientific literature, Smith and his colleague Ulrich Mueller found four documented cases of beneficial STIs in humans, aphids, mosquitoes and fungi.

STIs generally do less damage than other nasty bugs that are passed on through different routes such as air or water, Smith says. “Sexual transmission reduces how harmful microbes are,” he adds. This is because sexually transmitted microbes depend on their host’s chance of having sex to ensure the future of their own next generation. “One of the first things that animals do when they become infected with a pathogen is stop reproducing,” Smith says. The idea that some sexually transmitted infections could be good for your health is not such a stretch, he notes.

GB virus C seems genetically related to other viruses that cause hepatitis but GBV-C itself is not known to cause any disease in people at all. Its effect may actually prevent illness. Smith and Mueller’s review, published in the August issue of the journal Trends in Ecology & Evolution, covers evidence dating back to 1998 that GBV-C can cut the mortality rate in HIV positive people by 59 percent. GBV-C reduces the ability of HIV to enter and replicate in white blood cells, they wrote.

The GB virus alters the body’s immune response and dampens inflammation, says Jack Stapleton, an infectious diseases researcher at the University of Iowa.

Stapleton, who published a review of GBV-C in 2012, says the virus is very common in the community of microbes living inside humans. “If you are having enough sex to be exposed to HIV, it’s likely that you have also been exposed to GBV-C,” he says. Scientists have not yet harnessed GBV-C as a treatment for HIV, however, because of objections around deliberately infecting people with a virus, he notes. But he thinks the GBV-C could be used in combination with antiretroviral drugs to help save lives.

Smith and Mueller also found evidence of advantageous STIs in species other than humans. They found a 2006 study in Proceedings of the National Academy of Sciences showing that sexually transmitted bacteria passed from male to female aphids increased the insect’s resistance to parasites and high temperatures. The sexually transmitted bacteria (Hamiltonella defensa) secretes a toxin that guards against a parasitic wasp (Aphidius ervi), which lays eggs in aphids. Another sexually transmitted bacteria, Serratia symbiotica, makes aphids exposed to intense heat less likely to suffer cellular damage when their cells hold this bacteria. But scientists are not sure how this process works, Smith says. In another study researchers found that mosquitos sexually transmit good bacteria that helps their larvae develop more quickly, possibly by providing them with extra nutrition. Smith and Mueller also found evidence of viruses that boost the heat tolerance and growth rates of fungi. The viruses are passed on when the branchlike hyphae of one fungi fuses with that of another.

The more sex that individuals have, the more likely they are to pick up an advantageous STI. The increased survival rates could help explain the evolution of promiscuous behavior, which is widespread among the animal kingdom, Smith says. There are limits to this idea, however, says Joël White, an evolutionary ecologist at the University of Toulouse in France who was not part of this study. Other factors that also provide evolutionary advantages are likely to have a stronger influence, he notes. For example, mating with several partners increases the chances that females will become fertilized by a top-notch male.

There are drawbacks, too, that come with any possible advantages. For instance, promiscuous individuals are more likely to catch pathogenic infections, says Michael Lombardo, an evolutionary biologist at Grand Valley State University in Michigan, who also did not take part in the research. When the costs to a population are low over time, more promiscuity might evolve; if the costs are high, the behavior will be less likely, he says. “The drive to successfully reproduce via promiscuous behavior should be tempered by the potential costs, including pathogenic STIs,” he notes.

Smith agrees that beneficial STIs are likely to be just one among a host of influences on promiscuous behavior—but until recently they have been an unappreciated influence, he says. White also believes that advantageous STIs have largely gone unnoticed, possibly because the negative effects associated with mating are more noticeable than beneficial outcomes. “People unconsciously think of their own experience. We don’t know if we have caught a beneficial STI but people notice if they catch syphilis,” he says. Now that scientists have begun to look for beneficial infections, they might find more examples.