Until that system gets a shock.

“I didn’t sign up for a pandemic,” said Andrea Lindley, 34, an I.C.U. nurse at a Philadelphia hospital where scores of coronavirus patients have been admitted. “But I am not going to walk away when people need me.”

Growing up, she wanted to become a doctor, watching her mother come back exhausted and back-sore from long hours as a licensed practical nurse. Health care is harder physical work than people realize — workers in health care and social assistance suffer nonfatal injuries on the job at a rate higher than workers in construction or manufacturing. Ms. Lindley’s mother described the job to her this way: “You work too hard and you don’t get paid enough.”

But Ms. Lindley was attracted to the personal, hands-on practice of nursing. “We are in the rooms way more than the doctors,” she said. It is what she still loves about the job. These days, with her husband unable to find carpentry work and her daughter recovering from leukemia, it is also what makes the job so dangerous.

“I have horrible nightmares knowing I’m going into the hospital the next day,” she said. She felt a sense of deep relief when, on a recent shift, she was transferred to the burn unit.

Across the state, in southwestern Pennsylvania, Crystal Patterson heads to work. Her stepfather was laid off from his airport job, and her parents are unsure what they will do.

For Ms. Patterson, 30, a home health aide, there is less uncertainty. Yes, she has to manage caring for her son, but there is a client in her 90s who is depending on her. So for around $10 an hour, she stays on the job. There is a fundamental question before her, one faced by countless other women keeping the country alive: If she does not do this, who will?

“As a woman, this is nothing new to me,” Ms. Patterson said. “That’s how it’s always been in this country: ‘When we’re sick, get us through this.’”

Katy Reckdahl contributed reporting.

Methodology: The New York Times identified essential workers by applying the federal government’s essential worker guidelines with industry and occupation codes contained in U.S. Census American Community Survey microdata, 2014-18, obtained from ipums.org. In some cases, all workers in a category, such as law enforcement, were tagged as critical, but in other cases, such as retail, only workers for stores that have widely been allowed to stay open, such as supermarkets and drugstores, were included.