Parasite by Mira Grant
My rating: 3 of 5 stars
Summary: 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 tapeworm 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.