“Nurses often walked into scenes resembling those of the plague years of the fourteenth century,” wrote the historian Alfred W. Crosby in “America’s Forgotten Pandemic.” “One nurse found a husband dead in the same room where his wife lay with newly born twins. It had been twenty-four hours since the death and the births, and the wife had had no food but an apple which happened to lie within reach.”

In 1918, the world was a very different place, even without the disruptive influence of World War I. Doctors knew viruses existed but had never seen one — there were no electron microscopes, and the genetic material of viruses had not yet been discovered. Today, however, researchers not only know how to isolate a virus but can find its genetic sequence, test antiviral drugs and develop a vaccine.

In 1918, it was impossible to test people with mild symptoms so they could self-quarantine. And it was nearly impossible to do contact tracing because the flu seemed to infect — and panic — entire cities and communities all at once. Moreover, there was little protective equipment for health care workers, and the supportive care with respirators that can be provided to people very ill with coronavirus did not exist.

With a case fatality rate of at least 2.5 percent, the 1918 flu was far more deadly than ordinary flu, and it was so infectious that it spread widely, which meant the number of deaths soared.