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.