Signs of blood thickening and clotting were being detected in different organs by doctors from different specialties. This would turn out to be one of the alarming ways the virus ravages the body, as doctors there and elsewhere were starting to realize.

At Mount Sinai, nephrologists noticed kidney dialysis catheters getting plugged with clots. Pulmonologists monitoring COVID-19 patients on mechanical ventilators could see portions of lungs were oddly bloodless. Neurosurgeons confronted a surge in their usual caseload of strokes due to blood clots, the age of victims skewing younger, with at least half testing positive for the virus.

“It’s very striking how much this disease causes clots to form,” Dr. J Mocco, a Mount Sinai neurosurgeon, said in an interview, describing how some doctors think COVID-19, the illness caused by the coronavirus, is more than a lung disease. In some cases, Mocco said, a stroke was a young patient’s first symptom of COVID-19.