But the majority of Americans still cannot get tested, as interviews with doctors, patients, and dozens of state public-health officials reveal. While the most stringent federal guidelines are gone, a chaotic patchwork of rules now governs who can and cannot get a COVID-19 test. In many states, symptomatic patients still cannot get tested for the coronavirus unless they meet certain limited criteria—even if their doctor wants to test them.

Read: The strongest evidence yet that America has botched coronavirus testing

Those criteria mean that the cases most crucial to understanding the spread of the pandemic may never be discovered. In at least 13 states, the rules effectively discourage doctors from testing patients who have no known ties to existing cases—exactly the kind of “community case” that would signal that the pandemic has reached a dangerous new stage in a city or region, and that the virus is now spreading among strangers.

The rules are set by states, counties, and cities, as well as by individual hospitals and health systems. And while many are based on guidelines published by the CDC, their enforcement can vary profoundly on a state-by-state, and even hospital-by-hospital, basis.

Under the most widely used criteria, only people who have either traveled recently or have had known contact with a laboratory-confirmed COVID-19 patient can get tested, even if they have all the symptoms of the disease. This means that a city or region’s first community case may not qualify for a test, especially if the person is not sick enough to be hospitalized.

“If those are the requirements, you will miss almost all mild symptomatic transmission, and only become aware [that the coronavirus] is present in your community when it gets into a group of vulnerable people and starts killing them,” William Hanage, an epidemiology professor at Harvard, told me by text message.

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On a wider scale, the rules frustrate doctors, nurses, and other front-line medical workers as they try to take care of patients without endangering themselves. Even in the absence of local rules, doctors must sometimes involve dozens of people throughout a sprawling hospital bureaucracy before they can authorize a single test. The rules have prevented medical staff from getting themselves tested: In several cases, a doctor who had symptoms of COVID-19 told me they were denied a test because they could not prove they were exposed to a laboratory-confirmed case.

The rules almost certainly mean that the United States is still greatly understating the number of people nationwide who are sick with COVID-19, experts say. There are more than 1,800 discovered coronavirus cases in the United States, but estimates of the outbreak using statistical and genetic models suggest that thousands of people are already sick. An Ohio official estimated yesterday that 1 percent of the state’s residents—or approximately 100,000 people—already have the coronavirus, though she did not say how she reached that number.