Pardon me, but is that a worm crawling out of your butt?
Taenia saginata, the beef tapeworm, is a global parasite affecting millions of humans every day. The highest infection rate is currently found in Ethiopia, but with global travel increasing, the infection can be found pretty much anywhere.
Patients get infected with Taenia saginata by eating undercooked meat containing cysticerci, a larval stage of the worm which is contained in the muscles of infected cattle, deer and other herbivores. While in the jejunum, the worm develops fully and hooks itself to the lining of this part of the gut and starts feeding on the contents of your intestines. After some time it starts growing by adding new segments called proglottides to its body, until it contains a chain of about 2000 of them. An adult beef tapeworm can (according to the Center for Disease Control) grow up to 25 meters in your gut.
The segments at the end of the worms tail develop fertilized eggs, and - when mature - start leaving the host either passively with your stool or actively, crawling out of your anus.
Symptoms are normally pretty light, with some abdominal discomfort and - rarely - develop into intestinal obstruction. The other - more embarassing - symptoms are obviously the feeling of something forcing its way through your sphincter.
Treatment is fortunately easy with Niclosamide or Praziquantel, and after being killed, the worm is passed semidigested. If you want your guest in toto and measure it, you'd have to purge it with ca 2 l of saline after the drugs.
Dion R. Bell: Tropical Medicine, 4th Edition, Blackwell Science, 2000