Before the American Civil War, the majority of hospital nurses—or “stewards”—were men. But the war created a medical crisis that demanded more volunteers, and a lot of the people who took up the call were women.

Of the estimated 620,000 military deaths during the Civil War, about two-thirds were due to disease. If a bullet didn’t kill a soldier, the infection that developed from a wound might; and the infectious diseases that spread in war hospitals ravaged soldiers and medical workers alike. Amid this desperate need for medical workers, women began to volunteer as nurses for wounded soldiers. After the war, women continued to work in medicine; and by 1900, they represented 91 percent of U.S. nurses.

When the Civil War began in 1861, medical jobs weren’t yet professionalized as they are today, says Stanley Burns, a surgeon, historian and founder of The Burns Archive.

“Surgery was not part of medical training for many people,” he says. To become a doctor, “the only requirement was an apprenticeship with a doctor and some courses.” Many of the people who volunteered as surgeons during the Civil War essentially learned to operate on the job.

Similarly, there was no required training for the nurses who volunteered in war hospitals; so most of their training happened on the job, too. Although both Union and Confederate military medical departments preferred using men in war hospitals, the need for more nurses became obvious in the first few months of the war. Many of the men who ended up working as nurses in these hospitals were actually wounded soldiers who had been asked to help care for even more wounded soldiers.

Both white women and free Black women sought to fill this need by volunteering as nurses, though they had very different experiences. Free Black women were frequently assigned tasks viewed as more menial, and often could only treat Black soldiers or other nurses. In the Confederacy, slaveowners forced enslaved Black women to perform nursing duties and then slaveowners received compensation for the work.

American nurses working during the Civil War may have heard about the British nurse Florence Nightingale, who had emphasized the benefits of training for nurses during the 1850s Crimean War. She helped establish nursing as a profession in Britain, and influenced the way that some Americans began to think about nursing during the Civil War.

In 1861, the U.S. Army appointed Dorothea Dix as its first superintendent of nurses. Dix implemented a system for women to volunteer for three-month nursing assignments during the war. In addition to establishing standards of care for nurses who volunteered with the Army, she also helped shape the image of what a nurse should look like. To volunteer as a nurse under Dix, women had to be between the ages of 35 and 50, healthy and “plain-looking.”

Another influential Civil War nurse was the abolitionist Clara Barton, who became known as “Angel of the Battlefield” and went on to found the American Red Cross. In 1862, she made a harrowing journey by wagon to deliver medical supplies to the war hospital near Virginia’s Cedar Mountain battlefield.

“Five days and nights with three hours’ sleep—a narrow escape from capture—and some days of getting the wounded into hospitals at Washington,” she wrote of her journey. “And if you chance to feel that the positions I occupied were rough and unseemly for a woman—I can only reply that they were rough and unseemly for men.”

Thousands of women served as nurses during the Civil War, which served as a catalyst for more women entering the field. Like other medical jobs, nursing became more professionalized and specialized during the late 19th century. In 1873, New York City’s Bellevue Hospital opened the first U.S. nursing school based on the standards developed by Florence Nightingale. That year, hospitals in New Haven and Boston opened similar schools.

However, this professionalization also created a gendered hierarchy in pay and prestige. At the turn of the century, men made up the majority of doctors and surgeons, while increasingly more women held lower-paying nursing jobs, which were seen as less prestigious.

Today, nursing is the largest healthcare profession in the United States. Although more men have entered the field in the 21st century, women are still in the majority, making up 91 percent of nurses.