A tapeworn is a parasite that lives in an organ I cannot spell (sorry mate, mabye next time). You get them from numerious ways but there is no realistic way to prevent getting one (other than NOT eating expired food).
Basically, when you eat something, as it passes through your digestive system they will eat it when it comes to it, before your body can absorb any of the much neaded nutrients. Leaving it will eventually cause you to die, as the body needs what the tapeworm is taking.
You know you have one when you start loosing weight drastically, because the body is eating away at itself trying to stay alive. People usally start to suspect they a tapeworm when they never feel 'satisfied' or 'full' after eating anything, this is because you arn't, no matter how much you eat. But the best and most fool-proof way of finding out is by getting a doctor to examine you and take a few x-rays.
Good luck, I hope you do not have one.