Parasite

A decade in the future, humanity thrives in the absence of sickness and disease.We owe our good health to a humble parasite -- a genetically engineered tapeworm developed by the pioneering SymboGen Corporation. When implanted, the Intestinal Bodyguard worm protects us from illness, boosts our immune system -- even secretes designer drugs. It's been successful beyond the scientists' wildest dreams. Now, years on, almost every human being has a SymboGen tapeworm living within them.

But these parasites are getting restless. They want their own lives . . . and will do anything to get them. (Google Books Summary)