On a multicountry trip to South America, President Ronald Reagan couldn’t restrain himself from the inane observation that every tourist finds himself making about such trips. “Every country is different.” So, it seems, is every virus capable of spreading into a pandemic.

The great influenza of 1918-19, for example, tended to kill otherwise healthy people in the prime of life, ages 20 to 40. The COVID-19 virus tends to kill people age 70 and above, especially those with comorbidities.