When you think about sexually transmitted diseases, I'm willing to bet a housefly isn't your first thought.
As it turns out, flies can catch a fungus called Entomophthora muscae, which is a sexually transmitted disease afflicted houseflies with a special taste for necrophilia.
If flies had public service announcements, one of them would undoubtedly say "Entomophthora Kills!" In fact, this fungus always kills its host, and pretty quickly too.
This poses a certain problem for the fungus. How can the fungus spread from host to host if each new host dies so quickly?
Well, when the fungus kills a female fly, it continues to grow in her abdomen, causing it to swell. For a male fly, a swollen female abdomen is considered tremendously sexy. Under normal circumstances, these swollen abdomens are filled with fly eggs, a sign of fertility and readiness to mate. When a male fly sees that swollen abdomen, he doesn't stop to check the female's pulse, he just mates and flies away in search of other females.
This is how the fungus is carried from dead flies to living ones. Like an insect Don Juan, the infected male continues to mate with living females before he dies himself, spreading the housefly STD.