The young man admitted to Newton-Wellesley hospital in Massachusetts was clearly very ill - so sick that he needed the help of a ventilator to breathe.
The doctors who treated him were certain that he was infected with the new coronavirus. It was in mid-April - the culmination of the epidemic in the Boston area. The patient had difficulty breathing and had other symptoms.
But the first swab test for the coronavirus came back negative. A second test, 24 hours later, was also negative. So they tried a different approach.
"The clinical team asked me for help testing a tracheal aspirate," Dr. Michael Misialek, a pathologist at Newton-Wellesley Hospital outside Boston, told CNN.
It is a test that takes a sample from the deepest part of the respiratory system. He uses a different technique, but doctors are increasingly reporting that they cannot find evidence of Covid-19 infection until they have done these tests.
This tracheal secretion returned positive. "This allowed the team to enroll the patient in a clinical trial," said Misialek. He was treated with one of the many immunity-based drugs that were tried on patients with coronavirus, eventually recovered and left the hospital.
Similar cases were taking place around the world. In London, New York and elsewhere, health workers saw patients with severe symptoms - but then tested negative for Covid-19.
Why does this happen? Some studies are starting to indicate that when patients are seriously ill, the virus replicates deeper in the respiratory system, beyond the scope of the swabs used for most tests, experts say.
Dozens of tests are on the market, but their reliability varies considerably.
Polymerase chain reaction (PCR) tests, which look for evidence of the virus, are generally reliable - but not always.
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