There are over a thousand types of flat parasitic worms which can inhabit the gastrointestinal tract of vertebrates. But none of them is a welcome guest!
Tapeworms attach their scolex, or head, to their host’s intestinal system using hooks and suckers. From this head, a long, flat strobila comprised of numerous segments, or proglottids, is generated. Each of these segments is in fact a working organism, with an independent means of digestion and a hermaphroditic reproductive system.
As new segments develop, the chain grows longer and longer and eventually the old segments at the end of the tail break off, emerging from the host in a predictable manner. If they are fortunate enough to be ingested, their eggs can mature – typically in an intermediate host – into larvae, each of which can go on to achieve greatness of its own.
Tapeworms can live twenty years and grow to 50 feet long. (In fact, the Polygonoporus giganticus that makes its home inside of the whale gut can grow to 100 feet long!)
But although tapeworms often cause no symptoms, abdominal discomfort, fatigue, and loss of appetite are not uncommon. Fortunately, tapeworms can usually be treated with simple oral medication, often in a single dose.
Nevertheless, discretion is the better part of valor, and avoiding undercooked food is advisable, as it greatly reduces your risk of playing host to this unwelcome guest.