One doctor replied that one of his patients had a strange blood problem. Despite being put on anticoagulants, the patient was still developing clots. A second said she’d seen something similar. And a third. Soon, every person on the text chat had reported the same thing.

“That’s when we knew we had a huge problem,” said Coopersmith, a critical-care surgeon. As he checked with his counterparts at other medical centers, he became increasingly alarmed: “It was in as many as 20, 30 or 40 percent of their patients.”…

Increasingly, doctors also are reporting bizarre, unsettling cases that don’t seem to follow any of the textbooks they’ve trained on. They describe patients with startlingly low oxygen levels — so low that they would normally be unconscious or near death — talking and swiping on their phones. Asymptomatic pregnant women suddenly in cardiac arrest. Patients who by all conventional measures seem to have mild disease deteriorating within minutes and dying at home.

With no clear patterns in terms of age or chronic conditions, some scientists hypothesize that at least some of these abnormalities may be explained by severe changes in patients’ blood.