Ok, I'm a little late, so I figured I'd make this one extra-disgusting to cover the time you weren't cringing.
Meet the Screw Worm: Cochliomyia hominivorax. "Hominivorax" means "man-eater", just for your edification.
The screw worm actually isn't a worm at all - it's a fly larvae. Like other maggots, it likes to eat flesh. But unlike other maggots, it doesn't just eat dead flesh. It specializes on eating the nice, healthy flesh surrounding where it hatches, whether that be in an animals' wound or a newborn baby's bellybutton. They can't live off carrion like other maggots do.
So the mother, not to waste a good nursery, lays 200 or more eggs wherever she finds a nice, living host to nest in. The larvae which hatch then burrow into the surrounding flesh, eating whatever they come across. As they grow, they eat more and more. If they're disturbed, say, by a person scratching the infected area, they "screw in" deeper, hence their name. The more they eat, the more damage to the host, and the more festering, oozing sores attract more mama flies. Once the flies have grown enough, they climb out of the mess they created and drop into the soil to pupate.
They're a nasty parasite. In the US, luckily, they've been eradicated. Once prevalent all the way above Texas, the flies' last strongholds are in areas of South America, which the US and South American governments are still working hard to extinguish. Infestations can be treated with a larvicide to kill the offending guests and bandaged and treated to prevent more flies from finding the wound an enticing nursery. In areas where there is little or no healthcare, however, untreated humans and animals often die, if not from the worms themselves then from the wounds they inflict getting infected by bacteria. Some have claimed that screw worms can eat an entire sheep or dog from the inside out in five to seven days, kinda like a small and less dramatic version of Alien.