From New York Times bestselling author Tess Gerritsen, a terrifying novel of deadly proportions as an ER doctor tries to determine the cause and cure of a fatal and mysterious virus before it becomes an epidemic.

The quiet overnight shift at Springer Hospital ER suits Dr. Toby Harper just fine—until she admits a man in critical condition from a possible viral infection of the brain. The delirious man barely responds to treatment—and then disappears without a trace. Before Toby can find him, a second case occurs, revealing a terrifying fact: the virus can only be spread through direct tissue exchange. Following a trail of death that winds from a pregnant sixteen-year-old prostitute to her own home, Toby discovers the unthinkable: the epidemic didn't just happen—someone let it loose....