Parasite controls people as well as rats.

The common parasite, Toxoplasma gondii, hacks your immune system to make its niche. Previously reported, it also hacks rat brains to make cat pee smell sexy and sends rats fearlessly pursuing cats, thus turning them into dinner and allowing the parasite to reproduce in the cat's guts.  More than a third of all humans have been infected by the same parasite, though it varies by country.

Toxoplasma gondii parasites, green, multiply
inside an immune cell that lives in the brain.

"Toxoplasma hijacks immune cells to enforce a mutually beneficial balance," Denkers said. Toxoplasma quiets its host's alarm system by blocking immune cells from producing certain cytokines, proteins that stimulate inflammation," said Denkers. "Cytokines are double-edged swords: They summon the immune system's reinforcements, but if too many accumulate they can damage the body they're trying to defend. An unregulated immune response can kill you. When immune cells meet intruders, they release cytokines that summon more immune cells, which produce more cytokines, rapidly causing inflammation. T. gondii must allow cytokines to trigger enough of an immune response to keep its own numbers in check and ensure host survival." 

Side effects of the infection can include behavioural changes from recklessness to neuroticism, with links to schizophrenia and even sex ratios. I think more stories will follow that show parasites, bacteria and viruses infest people and strike a modus vivendi, give a little, take a little. See report that vesicular tomatitis virus suppresses some cancers in people.