In 1918, a new respiratory virus invaded the human population and killed between 50 million and 100 million people — adjusted for population, that would equal 220 million to 430 million people today. Late last year another new respiratory virus invaded the human population, and the reality of a pandemic is now upon us. Although clearly a serious threat to human health, it does not appear to be as deadly as the 1918 influenza pandemic. But it is far more lethal than 2009’s H1N1 (swine flu) pandemic, and the coronavirus does not resemble SARS, MERS or Ebola, all of which can be easily contained.

About 15 years ago, after yet another global contagion — the so-called bird flu — emerged in Asia, killing about 60 percent of the people it infected and threatening a catastrophic influenza pandemic, governments worldwide began to prepare for the worst. This effort included analyzing what happened in 1918 to identify public-health strategies to mitigate the impact of an outbreak. Since I had a historian’s knowledge of 1918 events, I was asked to serve on the initial working groups that recommended what became known as non-pharmaceutical interventions, that is, things to do when you don’t have drugs.

They involve only advice constantly reiterated today: social distancing, washing hands, coughing into elbows, staying home when sick. None alone provides great protection, but the hope was that if most people followed most of the advice most of the time, the interventions could significantly reduce the spread of the disease, or “flatten the curve,” a phrase now all too familiar. This may sound simple, but it is not. As with a diet, people know what to eat but often stray; here straying can kill.

As we begin employing these interventions now, we need to recognize what they can and cannot accomplish. Containment — the attempt to limit spread of a virus and even eliminate it — has failed. China has achieved far more than the most optimistic models predicted, but its initial slowness in responding allowed the virus to spread globally. Once that happened, the virus could not be stopped. Right now it is circulating invisibly in developed countries as well as in developing ones with little public-health infrastructure. That means it is here to stay and will constantly threaten to reinfect even countries that initially control it.