The biggest uncertainty now, experts said, is how many people around the world will die. SARS killed about 10 percent of those who got it, and MERS now kills about one of three.

The 1918 “Spanish flu” killed only about 2.5 percent of its victims — but because it infected so many people and medical care was much cruder then, 20 to 50 million died.

By contrast, the highly transmissible H1N1 “swine flu” pandemic of 2009 killed about 285,000, fewer than seasonal flu normally does, and had a relatively low fatality rate, estimated at .02 percent.

The mortality rate for known cases of the Wuhan coronavirus has been running about 2 percent, although that is likely to drop as more tests are done and more mild cases are found.