Last June, a sixty-year-old man in Saudi Arabia fell ill with pneumonia. His disease, it turned out, was caused by a virus no one had seen before. It was a coronavirus–in other words, it belonged to a lineage of viruses that includes ones that cause colds as well as ones that cause SARS. But this new virus was genetically distinct enough to be considered a species in its own right. Scientists now refer to it by the dreary, unpronounceable abbreviation HCoV-EMC. Eleven days after being admitted to a Jedda hospital, the man infected with this new virus died.

A single death from a new virus is hardly unheard of. But over the past few months, virus-watchers have gotten increasingly