“Where are the patients?” asked Dr. Steven Nissen, a cardiologist there. “That can’t be normal.”

One of the few was a man who lives in Cleveland. According to Dr. Nissen, the man felt chest pain while doing push-ups, but feared going to the hospital because there might be coronavirus patients there. He stayed home for a week, growing weaker — out of breath with the slightest exertion, his legs swelling. Finally, on April 16, he went to the Cleveland Clinic.

What should have been an easily treated heart attack had progressed to a life-threatening disaster. He survived after a dicey operation and spent nearly a week in intensive care, including several days on a ventilator, Dr. Nissen said.

The inpatient stroke unit at Stanford University Medical Center in California usually has 12 to 15 patients, said its director, Dr. Gregory Albers. On one recent day in April, there were none at all, something that had never happened.

“It’s frightening,” Dr. Albers said. Yet few Covid-19 patients have been admitted to the hospital, and people needing emergency treatment have little to fear.

“We prepared for an onslaught, but it has not arrived,” Dr. Albers said.

According to Dr. Samin Sharma, who heads the cardiac catheterization lab at Mount Sinai Hospital in New York, the number of heart attack patients fell from seven in February to three in March. So far in April there have been only two.

It’s not just the United States. Dr. Valentin Fuster, editor of the Journal of American College of Cardiology, said he is getting so many papers from around the world on the steep decline in heart attack patients in hospitals that he simply cannot publish them all.

A hospital in Jaipur, India, for example, that Dr. Sharma owns, treated 45 heart attack patients in January, he said. In February, there were 32, and in March, 12. In April, so far the number is just six.