Infection (Star Trek Next Generation: Double Helix)

Like the twisted strands of mutant chromosomes, an insidious alien conspiracy winds its way through the entire Alpha Quadrant, just as it stretches across several years of Starfleet history -- beginning near the very start of Captain Picard's command of the "Starship Enterprise(TM) !" It is only the first year of the "U.S.S. Enterprise(TM) "-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.