Contact tracing—mapping the spread of a virus by identifying individuals in the chain of transmission—is an essential tool in the fight to limit the damage of Covid-19. Early experience in South Korea, China, Singapore, Germany, and elsewhere has shown what works. Successful schemes save lives and mitigate economic losses. Smartphone apps have a role play, but as with e-commerce, security and privacy are concerns. Harvard’s Safra Center for Ethics recently published a paper outlining how contact tracing can be designed to protect users’ privacy. Coauthor Sham Kakade, a computer scientist at the University of Washington, explains.