There are 3 books in the Parasitology series. The newest book is Chimera which came out in 2015.

1
Parasite

Book cover of Parasite
From New York Times bestselling author Mira Grant comes a vision of a decade in the future, where 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…
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2
Symbiont

Book cover of Symbiont
THE ENEMY IS INSIDE US. The SymboGen-designed parasites were created to relieve humanity of disease and sickness. But the implants in the majority of the world's population began attacking their hosts, turning them into a ravenous horde. Now those who do not appear to be afflicted are being gathered for…

3
Chimera

Book cover of Chimera
The final book in Mira Grant's terrifying Parasitology trilogy. The outbreak has spread, tearing apart the foundations of society, as implanted tapeworms have turned their human hosts into a seemingly mindless mob. Sal and her family are trapped between bad and worse, and must find a way to compromise between…