On Monday's show, Tyra Banks profiled the tapeworm diet, which is supposedly the hot new thing among those in the know who want to stay slim.  Tapeworm diets have been the thing of fact and legend for at least a hundred years, when tapeworm eggs were sold as diet pills to the unsuspecting public.  (Welcome to a time before government regulation, here's your parasite!)  

Rumors about celebrities on tapeworm diets have circulated since time immemorial.  Urban legend debunking website Snopes covers the rumor, and has labeled it "Undetermined."  Opera singer Maria Callas is often rumored to have taken tapeworms deliberately to control her weight, but the truth is that she suffered from tapeworm infestation the honest way - by eating uncooked beef which had (unbeknownst to Callas) been contaminated with tapeworm eggs.

To her credit, Tyra apparently painted the tapeworm diet in a negative light, and emphasized that healthy exercise and a good diet is the best way to lose weight. She also brought on a medical expert, who produced a dead tapeworm in a jar of fixative to demonstrate what you're talking about when you're talking "tapeworm infestation."

Tapeworms are parasitic worms which infest virtually every kind of animal, from small to large.  Taenia saginata, the beef tapeworm, is one of the varieties most often encountered by people.  It can grow to be 40 feet long.  People may also accidentally ingest pork tapeworm and fish tapeworm from undercooked and contaminated pork and fish respectively.  

Tapeworm can also be spread through contact with unsanitary conditions, via either eggs or tapeworm segments in fecal material.  This contact can be either direct or indirect.  For example, if a restaurant worker infected with tapeworms fails to wash their hands thoroughly after using the bathroom, they can spread tapeworm through handling food served to restaurant patrons.

Tapeworms can also enter our homes via our cats and dogs.  Tapeworms can use fleas as an intermediary host.  The tapeworm infests the flea, and when Fido or Fluffy accidentally eat the flea while chewing at their itch, they get tapeworms.  The first symptom for a dog or cat infected with tapeworm is that they begin to shed tapeworm segments.  These look like small grains of white rice, and can be seen on the pet's stool and nether regions.

In the early stages of its life, the tapeworm eggs hatch into larvae, which circulate throughout the victim's bloodstream.  They colonize and form cysts in the internal organs, such as the lungs or liver.  

Tapeworm infection, known medically as taeniasis, often does not show any symptoms aside from weight loss and anemia.  Unless the larval tapeworms happen to encyst themselves somewhere inconvenient, like your brain or eyes.  The adult tapeworm fastens its mouth to the wall of your digestive system, and ingests the host's blood.  This slow blood loss eventually shows in reduced weight and condition.

Believe it or not, people are suckered into paying a great deal of money to receive a tapeworm infection.  Like a cat burglar disabling the burglar alarm, tapeworms (and all parasites) dampen down the host body's immune system response in order to survive.  According to some, this is a healthy counterpart to the overly (to their mind) hygienic world in which we live. 

Although it should be noted that if you feel your life is so clean that you feel your immune system is suffering, a better option is to take up a vigorous outdoor sport like rock climbing, or to volunteer to work with children.