President Trump has described his campaign rallies as “fun,” “wonderful,” “the Greatest Show on Earth,” and, of course, “BIG.” An effort to calculate whether those events have increased the spread of the coronavirus in the United States suggests that “contagious” and “deadly” would also apply. A rigorous attempt to gauge the after-effects of 18 of the president’s reelection rallies, all held in the midst of the COVID-19 pandemic, suggests they have led to more than 30,000 additional cases and at least 700 additional deaths. Those casualties would not have occurred if the campaign events had not taken place, according to...