Heart
The heart is a large, red, fleshy organ that, instead of playing music through pipes like it should, prefers to drench itself in blood and move it around inside of animals by beating against the inside of their ribcages.

The heart is said to be a parasite. This can easily be proved by physical exertion. When it's host has exerted themselves, the heart will beat wildly and sporadically, thus punishing their victims for trying to reject the parasite.

The best cure for the ill effects produced by this creature, is actually excersize and physical exertion. If you become fit enough, your body will gain the power to protect itself from the pain and adverse effects of the heart.
Person 1: Man, you look terrible, what happened?!
Person 2: I jogged 1k and now my chest hurts
Person 1: jeeze, you better get more active or your heart'll kill you