BoinBoing has a post about a cat parasite, Toxoplasma, that alters the brain chemistry of those infected with it. It supposedly makes infected rats easier to catch.

A parasite that causes rats to sacrifice themselves to cats may also change human behavior, making women more outgoing and warmhearted, and men more jealous and suspicious. The Toxoplasma bacteria is shed in cat feces, which are eaten by rats; infected rats become fearless in the presence of cats, which makes them easier to catch, which, in turn spreads the disease to new cats.

This explains everything.
Click the title link to see the original article at The Loom.