The estimated 45,000 intensive care unit beds in the United States would be swamped by even a moderate outbreak of about 200,000 in need of I.C.U. admission.

“I am very, very worried,” said Ashish K. Jha, who directs the Harvard Institute.

The strain is already playing out in unexpected ways. Since admitting the first known case in the United States, Dr. Keay’s hospital has followed Centers for Disease Control and Prevention protocols for sanitizing facilities used by multiple patients with suspected coronavirus infection, which resulted in slower treatment.

“If somebody had to go in for a chest X-ray, as most of these patients do, the X-ray room would need to be down for 30 minutes afterward to reverse the airflow and sanitize,” Dr. Keay said. “If you have lots of patients, that becomes a huge issue and delay.”