Here's something on tapeworms:
WHAT LEADS PEOPLE AND OTHER ANIMALS TO GET TAPEWORMS?
Early, immature forms of tapeworms can be present in some types of infected beef, pork, and fish. Cooking the fish and meat long enough will kill the tapeworms. Tapeworms are usually acquired when people eat meat or fish that has not been cooked enough. The tapeworms produce eggs in the intestines which then get passed out of the body in the feces (poop). If human feces wind up in the environment (due to poor sewage, for example) the tapeworm eggs may wind up in soil, grass, and water. When this happens, other animals, such as cows, dogs, pigs, and fish may consume the tapeworm eggs.
The tapeworms can then break out of the eggs when they are inside the animal's intestines. The first form of the tapeworm (known as a hexacanth) attaches itself to the wall of the intestine and continues to grow and develop. The tapeworm will develop into an adult when it is passed on to live in another organism. For example, the tapeworm can be in preadult form when it lives in a human and can develop into an adult form when it is passed on to a dog. From the above information, you can see that tapeworms can live in more than one animal at different points in their lives.
Another way that people acquire tapeworms is by touching feces that have tapeworm eggs in them and later putting their fingers in their mouths. Although it may seem like common sense not to do this, it can happen by accident.
WHAT ARE SIGNS AND SYMPTOMS OF HAVING TAPEWORMS?
Although beef, pork, and fish tapeworms can be very large, they rarely cause symptoms except for diarrhea and an upset stomach. Dwarf tapeworms also cause these symptoms. One sign of having tapeworms can be if pieces of the tapeworm break apart and come out through the anus (the area of the body that poop comes out of) or appear in the feces (poop).
Tapeworms can lead to cysts in an animal's muscles and other organs. A cyst is an abnormal lump, swelling, or sac that contains fluid, a part solid material, or a gas, and is covered with a membrane. A membrane is a thin layer of flexible tissue that covers something. The cysts can develop in organs such as the liver and lungs, a condition known as hydatid disease. The liver is the largest organ in the body and is responsible for filtering (removing) harmful chemical substances, producing important chemicals for the body, and other important functions. The lungs are two organs in the body that help people breathe.
From:
http://www.medfriendly.com/tapeworm.html