When Noopur Raje’s husband fell critically ill with Covid-19 in mid-March, she did not suspect that she too was infected with the virus.

Raje, an oncologist at Massachusetts General Hospital in Boston, had been caring for her sick husband for a week before driving him to an emergency centre with a persistently high fever. But after she herself had a diagnostic PCR test – which looks for traces of the Sars-CoV-2 virus DNA in saliva – she was astounded to find that the result was positive.

“My husband ended up very sick,” she says. “He was in intensive care for a day, and in hospital for 10 days…