The World Health Organization cautioned years ago that a mysterious “disease X” could spark an international contagion. The new coronavirus, with its ability to quickly morph from mild to deadly, is emerging as a contender.

From recent reports about the stealthy ways the so-called Covid-19 virus spreads and maims, a picture is emerging of an enigmatic pathogen whose effects are mainly mild, but which occasionally — and unpredictably — turns deadly in the second week. In less than three months, it’s infected about 77,000 people, mostly in China, and killed ...