There is a parasitic infection which 30% of the human population of this planet carries. It's easy to pick up, passes from mother to child, and can present no obvious symptoms. It has also evolved the ability to influence its host's thoughts, decisions, and behaviour.
Its traditional reproductive cycle is between cats and their prey.
Simple enough. An infected cat craps Toxoplasma, a rat eats the infected faecal matter and becomes infected, another cat eats the rat, and the cycle continues. Toxoplasma thrives in any warm-blooded animal, so human infection can occur through the faecal or flesh route, through eating or drinking contaminated food or water.
The cool (and/or terrifying) bit about this? Toxoplasma fucks with its host's head.
You see, rats are quite risk-averse. They have no defence against predators other than situational awareness, running really fast, and the ability to squeeze through tight escape routes. What Toxoplasma does is cause the rat's aversion to risk to decrease. The rat is bolder, more confident, and less likely to run when faced with a possible warning sign of a predator.
This makes the rat more likely to be eaten by the predator, which makes Toxoplasma spread more quickly.
Sucks to be a rat, right? Well, this effect isn't confined to rats. It happens to humans too. The parasite achieves its sneaky ends by causing massive secretion of a neurotransmitter called GABA – which rats and humans share in structure and function. The result is mental instability, reduction in risk-averseness, and aggression.
Two separate studies have shown that infected humans are 250% more likely to be involved in a car accident. Relative level of infection is directly correlated to increased suicide rates. Antipsychotics are as effective as antiparasitics for mood disorders in infected rats. Even significant cultural differences can be strongly linked to Toxoplasma infection rates.
Toxoplasmosis is dealt with by a cocktail of antiparasitics and antibiotics, which is unpleasant and expensive. For these reasons, infection is rarely treated unless severely symptomatic or in cases of immunosuppression such as in HIV+ people. Meanwhile, the little bastard sits in one in three human brains, tugging at the strings of our thought processes, and helping us make really bad decisions.