The spider bite
Possibly one of the more "believed" urban myths, this one tells the tale of a young person, often a traveller to a far-flung location, who is bitten by a spider and/or an ant. On returning home, the victim experiences a "hatching" whereby parasitic baby spiders and/or ants burst out from under their skin.
FYI – this isn't physically possible, but it hasn't stopped parasitology being a defining feature of the body horror genre from Alien and Wrath of Kahn to Stephen King's Dreamcatcher and Stephenie Meyer's The Host.