Parasites are scary and fascinating at the same time. It's disgusting to have a creature living and breeding inside another living animal, but the tricks they have to control their hosts can be surprising, weird and most of all scary.

There's been some talk about Toxoplasma gondii again because of this soon to be published paper. The paper describes how Toxoplasma gondii removes all fear of cats from rats infected with it. The reason for this is that the parasite only reproduces in cat bellies and of course the way to get there from the rat is to make it's host get eaten by one. The rats still had their normal anxiety and could learn to get frightened of other things.

It's strange how the the tiny parasite can control something so very specific. It's a common parasite too, about half of all people on earth are carrying it. It's not dangerous to humans unless you have a very weak immune system, like people with AIDS. Pregnant women should avoid getting it too. People get it the same way rats or other animals does, contact with cat litter.

Studies of humans infected with Toxoplasma have shown some subtle changes in personality. Quoting from this article:
Those infected, he found, show a small, but statistically significant, tendency to be more self-reproaching and insecure. Paradoxically, infected women, on average, tend to be more outgoing and warmhearted than controls, while infected men tend to be more jealous and suspicious.
There's not total consensus on this yet and it seems like only a few of all infected people have any notable issues with it. Toxoplasma infection has also been linked to schizophrenia, and medicine against schizophrenia made infected rats afraid of cats again. Why and how is not clear yet.

I would not be the slightest surprised if all of the truly cat-obsessed people are carrying Toxoplasma gondii.

Another classic example of a parasite controlling it's hosts is the Lancet Fluke. It has a complex three host cycle involving cattle, snails and ants. They live in sheep or cow livers where they mate and then leave their eggs out in the host’s feces. Snails eating that gets infected and to protect themselves they make slimy balls filled with parasites that they leave behind. Ants finding these slime balls eat them and get infected. Every night the fluke takes control over the and and makes it climb to the tip of a grass straw, hold onto it with it's mandibles and stay like that until the morning. Then it goes back to looking for food as normally, until the sun goes down. Finally, once some grazing animal accidentally eat the ant with the grass, the fluke is back in a host where it can reproduce.

There's more examples like this too. One fluke reproducing in wading birds makes infected fish shimmy and jump to make it easier for the birds to find and catch them. A certain hairworm who infect grasshoppers takes control of it's host when it is ready to leave it and makes the poor grasshopper suicide by jumping into some nearby water. The worm will be back in the water where it wants to be. (More hairworm here(pdf). Mind controlling parasites at LiveScience.)

I hope I'll never get a parasite infection. Especially not some slimy, crawling animal. Wikipedia has more information and links about this than you ever want to know...