Hold this for me please, will you? It came up in a discussion on homosexuality and I had to hunt it down again.
Toxoplasma gondi is a small intracellular protozoan parasite that can infect any warm-blooded animal. So says the Merck Manual, and imagine being the guy who proved _that_. It invades and multiplies asexually within the cytoplasm of nucleated host cells. With development of host immunity, multiplication slows and tissue cysts form. Sexual multiplication occurs in the intestinal cells of cats ( and apparently only cats) ; long-lasting oocytes are formed and shed in the stool.
Toxoplasmosis is dangerous in individuals with weak or incompletely developed immune systems. It can be devastating before birth, and is a big problem in people with AIDS. It likes to get into the brain - in animals other than cats.
This is a standard pattern - a parasite with a complex life cycle, that infects an intermediate host and enters the definite host ( cats ) when the definitive host eats the intermediate host. When this happens , manipulation of the intermediate host can help the parasite - manipulation that increases the chance of the intermediate host being eaten. I mentioned toxoplasmosis in a talk earlier this year and suggested that it might manipulate intermediate hosts, and it seems that it does. Recent work shows that uninfected rats fear and loathe the smell of cat urine. Infected rats are at best neutral - some actually seek out cat urine. Pretty obviously, toxoplasmosis is pushing buttons in the rat brain that increase its chances of ending up in a cat stomach. It doesn't appear to do much else - the rats act reasonably normally otherwise, don't look sick, etc.
I think that this supports two points I've been pushing - one, that persistent infections that don't cause obvious pathology may still be doing something, two, that we have to consider the possibility that infectious agents might alter behavior - including human behavior - as a part of their evolutionary strategy.
The third point is that having toxoplasma cysts in your brain may not be a good thing. Only a few billion people do. The infection rate varies a lot from place. It's about 20% in the U.S., about 87% in France ( which might explain a lot).
Thinking about this a bit more, it just might be that the state of being a cat-lover has a simple cause and cure.
"The presentation or 'gift' of the Holy Ghost simply confers upon a man the right to receive at any time, when he is worthy of it and desires it, the power and light of truth of the Holy Ghost, although he may often be left to his own spirit and judgment." --Joseph F. Smith (manual, p. 69)
"Inasmuch as ye have done it unto one of the least of these my brethren, ye have done it unto me."