Research indicates that bipolar disorder and schizophrenia are both caused by a parasite. Which disease you get depends on which part of the brain the parasite makes its nest: http://www.medicalnewstoday.com/articles/308233.php

This article only addresses schizophrenia, but it gives all you need to know about the parasite's ability to damage the brain. There are many ways to contract it, and Interestingly, it is passed via sperm. 50% of an infected man's offspring will be born with it (hence hereditary mental illness): http://www.theatlantic.com/magazine/archive/2012/03/how-your-cat-is-making-you-crazy/308873/