Star Trek the Next Generation: Double-Helix (Book 1) - Infection

It is only the first year of the U.S.S. Enterprise-D's ongoing mission when a virulent epidemic strikes the populace of Archaria III, endangering the lives of thousands and provoking acts of mob violence against those believed responsible for the spread of the disease.  While Data and Natasha Yar team up to uncover the true origins of the virus, Dr. Crusher finds that the implacable sickness resists all her efforts to find a cure.  The desperate quest for a cure becomes even more urgent when Deanna Troi succumbs to the dreaded plague...again and again.