Rat parasite makes cat pee smell sexy. Rat becomes cat food. Toxoplasma reproduces in cat.

Individual toxoplasma parasites (green)
are shown invading neurons (red) grown
 in a petri dish in the lab. The blue 
areas are fluorescently tagged cell nuclei.

Stanford University researchers have discovered that in male rats infected with the parasite Toxoplasma, the (sex pheromone region of the brain) responds just as strongly to the odor of cat urine as to female rats in heat.   The parasite preferentially infects the rat's amygdala (associated with emotional states).  Cat urine then smells sexy to rats and interferes with the host's instinct to flee.  Though Toxoplasma is only 1/100th of a millimeter long, it shapes the destiny of its host which is drawn to investigate the urine and get on the cat's dinner menu.
Science Daily News.

Added:  About 1/3 of all people are infected with Taxoplasma gondii too.  Hmmmmm.   Does this explain why people like to keep cats as pets?