The villain in this story is Toxoplasma gondii, a microscopic parasite that likes to take up temporary residence inside the brain and muscle tissue of warm-blooded animals. Once safely lodged inside the brain, Toxoplasma orchestrates a series of subtle neurological changes in the host. This process has numerous effects on the infected animal's behavior - effects that are especially pronounced in rodents. Afflicted mice and rats become fearless and disinhibited, and they exhibit particular boldness around housecats. Instead of avoiding cats at all costs (as any sane rodent would do), infected mice and rats will charge straight at a feline nemesis. It's a sure-fire strategy for winding up as cat chow, especially given the parasite's annoying tendency to induce slowed reaction times in its host.
If we didn't know any better, we might guess that Toxoplasma is trying to commandeer the rodent's body, to use it as a delivery vehicle to get itself inside a cat . . . and, in a very real sense, this is exactly what's going on. For it turns out that Toxoplasma can only reproduce effectively inside the feline small intestine. Its offspring can then hitch a ride back out into the world encased in cat feces, from which they make their way into the ground and then on into the body of any animal that happens to ingest them. Often the unsuspecting critter is something a cat can then prey on, after which the entire reproductive cycle repeats itself . . . Over the millenia, Toxoplasma has evolved a remarkable ability to make its way back inside cats to reproduce - primarily through altering the brain function of its temporary non-feline hosts.
Now, by this point in the story, some of you are no doubt wondering, "Hey, wait a minute; haven't I heard of this parasite somewhere before? Isn't it the one that causes toxoplasmosis, that disease pregnant women can catch from cats and pass on to their babies?"
Indeed it is. Fortunately, though, the best research suggests that Toxoplasma infection is no more likely among cat owners than it is among the general population, so the risk of contact with cats appears to be minimal. Medical experts do, however, urge some caution when cleaning out a litter box, as Toxoplasma is readily transmitted to humans through the accidental ingestion of cat feces. (As if such an unsavory fate weren't bad enough!)
But if people aren't typically contracting Toxoplasma from contact with cats, how are they getting it? The biggest culprit is the eating of undercooked meat. As we've seen, Toxoplasma can lodge itself in the muscle tissue (i.e., meat) of any warm-blooded animal, including pigs, cattle, sheep, and chickens. A non-trivial portion of our meat supply is infected. Luckily, heating tainted meat to at least 155 degrees Fahrenheit (67 degrees C) for a few minutes appears to be sufficient to prevent infection. Obviously, to be completely safe, you might want to consider going even hotter and longer . . .
1) Just as Toxoplasma reduces reaction times in rodents - presumably, to make them easier for cats to catch - it also makes people react more slowly. In fact, infected individuals are at much higher risk of experiencing an automobile accident. (And remember, most people who have contracted Toxoplasma are completely oblivious to the fact.)
2) Women infected with Toxoplasma experience a suite of changes that might be considered generally positive. They tend to be very kindly, and to exhibit, on average, slightly higher IQ, conscientiousness, warmth, generosity, and guilt-proneness. Why in the world would the parasite engender these particular effects? Well, anything Toxoplasma can do to aid and abet its genetic kin will be subject to evolutionary selection pressure, since this will still effectively help pass along its genes. Thus, Toxoplasma appears to be trying to turn its female hosts into people who will be particularly nice to cats - aka, "cat ladies!"
3) Men, on the other hand, have a completely different set of reactions to Toxoplasma infection. They experience reduced IQ and initiative, and tend to become more 