A couple years ago a read a book called, Parasite Rex by Carl Zimmer. It was an excellent book on parasites and how they affect the human body. Living in America we tend to forget all those nasty critters in some areas of the world.
Some how I came across his blog and found a fascinating post about how scientists had found that a single cell parasite that lives in cats was able to change the normal fear reaction of rats. What was more interesting was that by administering schizophrenia drugs to these rats their fear came back regarding cats. I've clearly oversimplified this post, but to read more click here for his Return of the Puppet Masters.
Think about it - how many diseases are there that are caused by viruses or bacteria? Stomach ulcers and cervical cancer are two examples.