While I love to take these opertunities to mock my cat owning friends, the interesting thing is there is growing suspicion that many mental illnesses might be caused by verious infections of one type or another. A couple of long known examples of this are syphilis causing psychosis and bacteria from ticks causing lyme disease. And even research showing a possible connection between herpes and bipolar disorder.
This might allow for new and better treatment and possibly vaccines for mental illnesses.