Imagine two rats – they’re identical in every way. They’re the same size with the same face, same fur. Except one is infected. A worm, a specimen of Toxoplasmosis, has burrowed it’s way into the rat’s brain. But you can’t see that, the two appear the same.
The two rats move along, hugging the basement wall. There’s a lot of things a rat fears. They shy away from the light that comes from the uncovered bulbs. They avoid loud noises. There’s a cat’s litter box in one corner of the basement and, normally, both would keep well away from it. Rats have evolved over millions of years to recognize the scent of a cat and fear it. But one, the infected one, doesn’t. Instead it moves closer.
The parasite is responsible for this. It used to be thought that parasites passively leeched off their hosts, but parasites can actually drastically affect their host, even influencing their behavior. The horsehair worm drives its hosts to drown themselves. The cordyceps fungus drives ants and other insects to climb. And Toxoplasmosis turns off a rat’s fear of cats. Toxoplasmosis does this by congregating in the rat’s amygdala, the emotional lynchpin of the rat’s brain, and emitting a specific hormone called tyrosine hydroxylase. This hormone stimulates the production of dopamine, the brain’s feel-good chemical. It’s as if the parasite is loading the rat’s brain with cocaine. It’s mind starts to become unwired.
Instead of running away, the rat ultimately becomes attracted to the cat’s scent. And that’s what the parasite wants. Toxoplasmosis can only breed inside a cat’s gut, so in order to complete it’s life cycle it has to get the rat eaten. And given enough time, it will.
The cycle isn’t always so neat, however, and Toxoplasmosis can infect other animals, including humans. People with weakened immune systems have to be careful, particularly HIV-positive people and pregnant women, but most people can shrug off an infection with nothing more than a headache. Although, consider this. The parasite can’t tell you’re not rodential – it will still produce tyrosine hydroxylase, regardless of its host. Which still affects your brain. And if preliminary research is to be believed, it can affect your mind just as much as a rats, causing a variety of symptoms, including gregariousness, paranoia, aggression, and even schizophrenia. Fun!