In Maryland, the pandemic is straining an already overtaxed volunteer emergency response system. Before the virus, the Wheaton Volunteer Rescue Squad in Montgomery County was among the nation’s busiest ambulances services, said Gamunu Wijetunge, the squad president.

On nights and weekends, all-volunteer crews of nine handle emergency calls. They pull 12- to 18-hour shifts, and many of them are off-duty firefighters and medical professionals.

As cases continue to climb in Montgomery County — there were more than 3,480 cases and 152 deaths as of Saturday, the second-highest count in the state — the squad has tried increasing its dwindling ranks of 100 or so volunteers, Mr. Wijetunge said, while also barring dozens of recruits who have not completed E.M.T. training.

But he said the squad’s efforts to meet minimum staffing requirements have been hindered by local competition: a children’s hospital and a medical school that have told employees they cannot volunteer for shifts during the pandemic.

Maryland’s governor announced this month that people who had worked as E.M.T.s or paramedics in the past decade would be granted provisional licenses, a welcome relief for Eric Bernard, president of the Rockville Volunteer Fire Department.

More than 10 percent of the 160 volunteers have stopped taking shifts, he said. Some have exhibited coronavirus symptoms, but others are worried their underlying medical conditions could put them at greater risk.

The ranks have also been diminished by the loss of firefighters too busy with their day jobs and medical professionals forbidden from volunteering by their employers. Mr. Bernard fears those numbers will grow. “We’re nowhere near toward the end of this,” he said.