For weeks, first responders have been racing across New York City to try to save lives in the national epicenter of the coronavirus pandemic.

The stress from the high numbers of 911 calls and deaths from COVID-19 is taking a toll on them.

Elizabeth Bonilla, a paramedic for the New York City Fire Department, said every itch or scratch in her throat, minor headache or sneeze has her worried. Bonilla said she can't help but wonder, "Could I be next?"

The question gnaws at her, especially when she's at home in her apartment in the Bronx, trying to sleep between what are often 16-hour shifts. She's started leaving the light on in her bedroom.

"It's hard for me to fall asleep in the dark because I get the image of that lady or that man who either passed away or who's suffering," Bonilla said. "I hear the cries. I hear the agony of people suffocating, trying to breathe."

Responding to calls, Bonilla said, feels like coming face to face with a monster that's overtaken a patient's body.

"It's almost like the virus is talking to you. As soon as you see a patient, it's like, 'Oh, yeah, this person has the virus,' " she added. "They're pale in the face. Some of them are blue in the lips, blue in the fingers, blue in their toes. They're panting like a dog."

Seeing one patient after another go into cardiac arrest, it's easy to feel helpless, Bonilla said.

"You have just your little tools in your little bag," she said. "Most of our medications — I want to say all of our medications — don't even work on the patient."

At the height of the crisis in March, the daily number of 911 calls in New York City skyrocketed, increasing by as much as 50% above the usual call volume, according to city fire department spokesperson Frank Dwyer.

This month, the number of emergency calls has fallen, but Mayor Bill de Blasio said the city's emergency medical service still needs the extra support from all of the ambulances, as well as out-of-state paramedics and EMTs, that FEMA has sent — and will need them through the end of May.

"There's no question that we are not out of the woods yet," de Blasio said during a press conference on Monday.

Vincent Variale, president of the Uniformed EMS Officers Union, Local 3621, which represents some of the city's emergency medical services workers, compares the worst days of the pandemic to some of New York's darkest moments.

"This is like 9/11 happening every day," Variale said.

On Monday, the mayor announced around 18% of the fire department's EMS workforce was out sick, including those diagnosed with COVID-19.

"We were already understaffed to begin with, but now we have more people out sick and we have an increase in the call volume," Variale said. "This is a recipe for disaster."

Don Goepfert, an EMT who recently taught at the city's fire academy, tried to plan ahead with his family before the outbreak hit New York. In March, his wife and three children left their home in Brooklyn to stay with relatives in Pennsylvania, which, Goepfert said, was "not the easiest choice, but the best choice."

"Some people are sleeping in their cars and not even going home or sleeping in the basement where they live," Goepfert said about EMS colleagues who are trying to avoid infecting their families with the coronavirus.

Living alone, though, made Goepfert extra nervous after he came home from a shift and started feeling body aches and his temperature rising. That night, Goepfert said, he made sure to leave the front door unlocked in case EMTs had to get in.

"I had that anxiety in me," he said. "What if I don't make it through the next day? So I was trying to prepare for the worst."

Last week, Goepfert was admitted to a hospital after he was diagnosed recently with COVID-19.

"I'm kind of upset that I'm sick because I want to be out there," Goepfert said. "It's our time to shine and prove to the city that our services are much needed."

For Bonilla, the paramedic, there is some comfort these days at 7 p.m. in the evening, when shut-in New Yorkers open their doors and windows before the sun sets for the city's nightly salute to first responders, hospital staff and other essential workers.

Hearing her neighbors show their appreciation by cheering, honking their car horns, banging on pots and shooting off a few fireworks was "very warming" on a recent night off, Bonilla said.

"It's like a battery to your back," she said. "It makes you want to keep going."

That night, her two sons watched the show of appreciation with her from a window in their apartment. It's because of them she's spending most of her time at home inside her bedroom, alone and worried about exposing them to the virus that she and some 4,400 EMS workers with New York City's fire department are dealing with each day.

"We can only wish to spend unlimited time with our family, to have dinner at the table with our family and be able to share movie time and stuff like that with our children," she said. "But we don't have that option. We take it as we can."

When Bonilla has trouble falling asleep, she turns to gospel songs and yoga music.

And in her bedroom, for now, she's still keeping the light on.

NPR science reporter Rebecca Hersher and North Country Public Radio reporter Brian Mann contributed to this report.

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