Viruses are notorious for evading man-made drugs, but they are powerless against a rare mutation in the gene ISG15. A person who possesses this mutation is better at fighting off most (if not all) of the viruses that plague humankind—but probably fewer than one person in 10 million has it. Dusan Bogunovic of the Icahn School of Medicine at Mount Sinai thinks it may be possible to develop a drug that mimics this mutation. If he is right, he may be on the verge of developing a pill that will temporarily bestow the ability to fight off a virus—any virus—without getting sick. The medication should also grant the person who takes it lifelong immunity to any strain encountered while on the drug (unless the virus mutates, as the flu does).