A common single-celled organism called Toxoplasma gondii is a parasite that infects many animals, including rats — in which it causes a peculiar and unexpected transformation. We all know that the average rat will avoid cats at all costs, and they become particularly nervous when exposed to cat urine. That is, unless they’re infected with Toxoplasma.
Research has demonstrated that infected rats abandon their normal fear response, and instead go exploring. It’s clearly bad news for the rat, but splendid news for the parasite — Toxoplasma reproduces sexually only in cats. Therefore, the hunter cat is quite helpful with the parasite’s reproduction.
The parasite can also infect humans, in which it causes toxoplasmosis — one of the reasons pregnant women and immunocompromised people should avoid contact with cat droppings.