After a painfully slow rollout of diagnostic testing for active coronavirus infections across the country, some 400,000 people a day in the United States may now receive such a test, estimates suggest. Yet a few public health experts say sending people back to work and school safely and identifying new outbreaks before they spread out of control could require testing much of the U.S. population of 330 million every day. Others suggest checking roughly 900,000 people per day would be enough. Robert F. Service writes in Science that either way, nearly all the current tests to diagnose infections work by identifying the genetic material of the virus, a technology that will be difficult to scale up much further.