Day 2: A monster that brings about a specific disease.

It didn't specify mental, bacterial, or viral disease -- so I picked bipolar disorder. This demon influences moods and feeds off of them regardless of being good or bad. It's hunger is unquenchable, and will often make the lives of those inflicted their own personal Hell. Medication probably wouldn't repel him, only make the moods more tolerable.

Another one for the 28 day monster challenge.