Imagine an infection that made you enthusiastically seek out the presence of grizzly bears, essentially offering yourself up as a tasty snack. Sounds ridiculous or even impossible, right? Oddly enough, it is entirely plausible. The parasite Toxoplasma gondii - a mere single-celled organism - is able to manipulate the brain responses of rodents to the odour of cat urine. The usual fear response is replaced by sexual attraction, so causing individuals to actively seek out areas where they are likely to run into their predators. Toxoplasma gondii requires the feline digestive system in order to sexually reproduce, so by modifying the rodent’s behaviour to cause it to spend more time around cats, it increases the likelihood of the individual ending up exactly where they need it to be - in its stomach.
Ref: House, Vyas & Sapolsky (2011) Predator Cat Odors Activate Sexual Arousal Pathways in Brains of Toxoplasma gondii Infected Rats. PLoS ONE 6(8): e23277.