Lovely question.
I plan on going into research science some day, and I'd say I'd work with either cancer or autism. Probably autism, as it affects me on a daily basis (I'm high-functioning/Aspergian myself, and have to deal with things such as facial expression recognition and the like all the time. Plus two of my good friends have low-functioning autism, so it's around me a lot).
Then again, life is so difficult, spontaneuos, and unreasonable, that I think it might be the thing that needs looking into. XL