This is a great question, and the answer may surprise you. The hygiene hypothesis postulates that exposure to multicellular parasites like tapeworms is actually a necessary step in the development of the human immune system. The idea is that, as a child's immune system develops B and T lymphocytes, those lymphocytes go into action creating antibodies; if there are no parasites or pathogens present to create antibodies against, the immune system may create antibodies against self tissue. This can lead to allergies or autoimmune diseases such as type I diabetes, asthma, and lupus. Researchers have found a strong negative correlation between parasite infestation and autoimmune diseases, which supports the hygiene hypothesis.