Third straight biological unpleasantry by redeployed internist Gerritsen (Support, 1997, etc.), who shuns no commercial device on the quest to suck in ever more readers. While fairly deft at characterization, Gerritsen shows even more smarts in chronicling high-energy ER work (she writes about more bodies than doctors) and ingeniously invents a new brain-altering parasite that has spread through the small resort town of Tranquility, Maine. Dr. Claire Elliot has moved with her adolescent son Noah to Tranquility’s Locust Lake after he failed to come to grips with his father’s death in Baltimore. As the replacement for the late Dr. Pomeroy, she has her problems being accepted by the locals. But these are nothing beside the plague of violence erupting among the town’s young folk. Their mysterious rages bring on many deaths, including the shooting of Noah’s biology teacher during class and such bloody events as all-nails-bared catfights among predatory girl students. Slowly, Dr. Elliot comes to believe in her theory of a parasite invading Locust Lake, where the kids swim. Of course, the townies, who have read their Ibsen (or Peter Benchley’s Jaws), won’t hear of this—the bad news would demean Tranquility’s resort attractiveness—and blame the messenger for her insights. Well, is the villain really a pork tapeworm whose eggs were blithely flushed into the lake? Did yet other eggs cause Tranquility’s none-too-tranquil murder rampages 50 and 100 years ago? Does the globby green bioluminescence that appears on Locust Lake—and even smears the violent kids—point up the source of the mystery parasite? And what’s this green glowing earthworm in Noah’s sinus that’s nearly killing him? Weirdly terrific stuff with a steel grip. Doc Gerritsen’s mastery of the stupefying stupidity inherent in adolescent resistance to almost everything may well give the reader gray hairs.