Toxoplasma gondii can infect humans as well* if they have sufficient contact with cat excreta, and lead to a similar disregard for dangerous situations. I conjecture that some non-negligible fraction of violent criminals carry this parasite and that it would be worth screening for in incarcerated populations.

* http://www.theatlantic.com/magazine/archive/2012/03/how-your...

"The virus and the wasp have the same evolutionary interests; turning a ladybug into a bodyguard produces more wasps, and more wasps beget more viruses. And so their genes work together to make the ladybug their puppet. The D. coccinellae wasp may not be the puppet master it once seemed. Instead it hides another puppet master within." Great now I just can't sleep! Haha great read.

Pop-ups.