According to the Tracking Project’s figures, nearly one in five people who get tested for the coronavirus in the United States is found to have it. In other words, the country has what is called a “test-positivity rate” of nearly 20 percent.

That is “very high,” Jason Andrews, an infectious-disease professor at Stanford, told us. Such a high test-positivity rate almost certainly means that the U.S. is not testing everyone who has been infected with the pathogen, because it implies that doctors are testing only people with a very high probability of having the infection. People with milder symptoms, to say nothing of those with none at all, are going undercounted. Countries that test broadly should encounter far more people who are not infected than people who are, so their test-positivity rate should be lower.

The positivity rate is not the same as the proportion of COVID-19 cases in the American population at large, a metric called “prevalence.”* Nobody knows the true number of Americans who have been exposed to or infected with the coronavirus, though attempts to produce much sharper estimates of that figure through blood testing are under way. Prevalence is a crucial number for epidemiologists, in part because it lets them calculate a pathogen’s true infection-fatality rate: the number of people who die after becoming infected.