One risk of sexual behaviour is catching a disease from a partner. But sex partners trade a lot of other micro-organisms as well, and some of them might actually have beneficial effects.
Natasha Gilbert, writing for Scientific American, reports on a human virus called GBV-C which seems to improve survival in HIV-positive people.
It's one of four known sexually transmitted infections that seem to help their animal hosts, and evolutionary ecologists Chad Smith and Ulrich Mueller from the University of Texas, Austin think that if biologists start looking, there might be even more.
From the article:
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.
It's not that some sexually transmitted diseases aren't awful, or deadly. But out of all the microbes that live on or inside animals, they may just be the ones we notice most.