There is a parasitic bug called toxopasma gondii, found primarily in raw meat, that may be the reason you flipped off that guy on the freeway the other day. Link
The bug is really only "active" for a few weeks after contracting it. Though it does give you some flu-like symptoms, they are subtle enough that many people never even realize they're sick. After a few weeks, the bug goes into a long-term dormant stage.
There is mounting evidence that in this dormant stage people's behavior can be altered due to a change in the chemistry of their brains that leads them to be aggressive, reckless, and impulsive.
A study of nearly 4,000 military drivers in 2009 found that those with latent infections are up to six times more likely to have been involved in a crash. There have been countless studies between 1959 and today that seem to corroborate this conclusion.
But...wouldn't putting a host into impulsive rages be a bad thing for the bug? Well, toxopasma gondii can live in any warm-blooded animal but, for whatever reason, can only reproduce in cats. Thus, the bug may want a chance to kill us and escape and find a cat to reproduce in, thus keeping their linage alive.
Currently, there is no medicine to treat the effect this bug has on peoples' brains but there is a strong push by professors of neuroscience to develop them.