Doctors are reporting bizarre, unsettling cases. 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.

Some of these abnormalities may be explained by severe changes in patients' blood.

Some doctor groups have raised the controversial possibility of giving preventive blood thinners to everyone with covid-19 – even those well enough to endure their illness at home.

COVID-19 autopsies have shown some people's lungs filled with hundreds of microclots.

Clots might be responsible for a significant share of U.S. deaths from covid-19.

A Dutch study published April 10 in the journal Thrombosis Research provided more evidence the issue is widespread, finding 38% of 184 covid-19 patients in an intensive care unit had blood that clotted abnormally. The researchers called it "a conservative estimation" because many of the patients were still hospitalized and at risk of further complications.