When YouTube user Brent Askwith saw a freakishly large worm slither out of a spider he had just killed, he recorded the ghoulish event and appropriately named the video "WTF IS THIS?!?"
"I was just editing my latest montage and this huge spider came out, so I sprayed it and killed it, then this fricken alien worm came out," Askwith wrote in the video's description.
That "alien worm" is actually a parasitic nematode, also known as a roundworm. While the nematode in the YouTube video is larger than most, Harvard University entomologist Dr. Brian Farrell told The Huffington Post that every human is infested with thousands of tiny nematodes.
"Most have no obvious effect on us, and we are mostly unaware of their presence," he wrote in an e-mail, "but a few are large enough to cause diseases such as trichinosis."
In addition to looking strange, nematode parasites can cause their hosts to do strange things. Dr. Farrell gave the example of some nematodes that prey on ants -- the parasite makes its host climb a tree and wave its butt in the air in order to catch the eye of a bird. The bird then nabs the ant, allowing the parasite to escape through the ant's abdomen and spread to other potential hosts.
"My personal favorite is Toxoplasma gondii," Dr. Farrell wrote, "the protozoan that infects cats (and is the reason pregnant women should not be around cats). Toxoplasma also infects rats and makes them unafraid of cats, so they get eaten and the parasites are able to then infect the cats they desire. Weird."