Rabies
Transmitted by rabid animals, you're most commonly found infecting creatures such as raccoons, skunks, bats and foxes. But don't worry, you affect humans too, causing either paralysis or hyperactivity in your advanced stages, and ultimately death.
Your most famous symptom is hypersalviation - that delightful foaming at the mouth that we have come to know and indeed love. However, you can also cause hallucination; think of the fun you could have at parties!