Try to repackage it as a diet aid all you want, but a tapeworm is a parasite through and through. It obtains all its nutrients from the digestive system of its host animal. But here's a strange fact – a tapeworm doesn't even have a digestive system of its own. It absorbs nutrients (mostly carbohydrates and sugars) that have already been partially broken down by the host's digestive system [source: Zimmer].
An adult tapeworm lives inside the intestines of the host animal, which could be a pig, cow, dog, sheep, fish or even a human. Different species of worm prefer different hosts, although most can infect several different types of animals. At the top of the worm is the head, called the scolex. The scolex has suckers or hooks that it uses to attach to the intestinal wall. Without this feature, the peristaltic action in the host's intestines – the rhythmic muscular contractions that move material through the digestive tract – would push the worm out. Below the scolex is the neck. The rest of the worm's body buds off of the neck [source: Encyclopedia.com].
Here where it starts to get really creepy. The tapeworm's body, called the strobila, is made up of many segments, sometimes thousands. Each individual segment is known as a proglottid (pronounced proh-GLAH-tuhd). A proglottid is either male or female. The segments closest to the neck are mostly male and produce sperm. Segments farther along the body are female and are basically egg sacks. A tapeworm can produce millions of fertilized eggs. In fact, each individual body segment is home to about 100,000 eggs [source: Bradford].
Proglottids frequently break off from the worm and are passed out of the host along with feces. However, each proglottid has functional muscles, allowing it to crawl. Sometimes, a detached proglottid will crawl out of the host through the anus. More often, a proglottid crawls away from the pile of fecal matter left by the host, increasing the chances that it will be eaten by a different animal. Eventually, the proglottid disintegrates, leaving behind the eggs.
When the eggs are eaten by an herbivore, they are "hatched" by the conditions within this intermediate host's digestive system. The larval stage, known as a hexacanth, uses hooks to burrow through the host's intestinal walls to reach the bloodstream. There, it turns into a scolex and forms a cyst. The resulting condition is known as cysticercosis. Pigs, cows and sheep are most commonly infected. Humans can act as an intermediate host for the pig-born species of tapeworm, and therefore can contract cysticercosis [source: Pearson]. We'll get back to the horrors of cysticercosis in a minute.
Once the cysts form, the parasite simply waits within the host. When the host animal dies, the meat may be eaten by other animals (or by humans) raw or undercooked. The ingested cysts pass into the primary host's digestive system, where the scolex attaches to the intestine wall and the whole cycle begins again.
Once a person has a tapeworm, they're officially saddled with a parasitic infection known as "human taeniasis." On its own, human taeniasis isn't such a big deal, as it is often asymptomatic. However, it can develop into cysticercosis, the condition resulting from tapeworm hexacanths burrowing their way into your bloodstream, which is decidedly not pleasant. The cysts can end up pretty much anywhere in your body, including in your eyes or your brain. The cysts sometimes grow, and they inflame the surrounding tissue. The resulting pressure can cause temporary symptoms or permanent damage, including blindness, brain damage or even death in some extreme cases. In developing countries it's considered to be the main cause of adult-onset seizures, and is far more likely to happen to people who are living with someone who has a tapeworm already [sources: CDC, CDC].
Tapeworms can also wreak digestive havoc if they get really big and block the appendix, bile ducts or pancreatic ducts. Tapeworm cysts can also grow within organs like the lungs and liver, and if they get large enough they'll disrupt the functioning of said organs. Occasionally, one of these cysts will even rupture, which can cause itching, hives, trouble breathing and swelling not unlike a severe allergic reaction [source: Mayo Clinic].