The cause was complications from a heart attack, said a niece, Doris Kressly.
The daughter of Salvation Army officers, Gen. Hays had dreamed of becoming a nurse since she was a young girl, wrapping bandages around the legs of a kitchen table where her parents frequently invited the infirm to dinner. She went on, in Vietnam, to oversee a 4,500-person nursing corps whose robust use of antibiotics, whole-blood transfusions and speedy helicopter evacuations was unforeseen when she began assisting doctors at a dirt-floored hospital in Ledo, India, in January 1943.
Working in a malaria-infested stretch of the China-Burma-India theater, she treated gangrenous construction workers who were building a new roadway that supplied the Chinese military in its war against Japan, as well as lice-infested members of the special-operations Army unit known as Merrill's Marauders.
Most of her staff was sick with malaria, dysentery, or dengue fever, she later said in an Army oral history, and at one time she found herself hospitalized and spotted a cobra under her bed. She calmly asked a guard to shoot it, later explaining, "When one lives in the jungle, one can expect that sort of thing."
Gen. Hays treated some of the earliest casualties of the Korean War, helping establish the first military hospital in Inchon, and as chief of the Army Nurse Corps from 1967 until her retirement four years later, she helped bolster its ranks during the conflict in Vietnam.
As part of an effort to expand scholarship opportunities and educational requirements for Army nurses, Gen. Hays "persuaded the Army that nursing was important enough to spend money on — a hard sell at that time," said Sanders Marble, senior historian in the Army's Office of Medical History.
Notable deaths in 2017
She also helped push through Army policy changes that paved the way for women in the military, including the 1970 establishment of maternity leave for female officers. Through her efforts, married officers were no longer automatically discharged from the ranks for becoming pregnant, and a provision was removed that limited mothers' ability to join the Army Nurse Corps Reserve.
Gen. Hays resisted a close association with feminism — "Let's not talk about this," she told the New York Times in 1970, when asked about the burgeoning women's liberation movement. But she nonetheless became a symbol of unprecedented female advancement on June 11, 1970, when she was promoted to the one-star rank of brigadier general.
Until three years earlier, the rank had been barred to her by law. Legislation under President Lyndon B. Johnson opened up the possibility of a female general — the first "in the Western world since Joan of Arc," Gen. William C. Westmoreland said — and in 1970 President Richard M. Nixon made good on the new rules, selecting for promotion Gen. Hays and Elizabeth P. Hoisington, chief of the Women's Army Corps.
Whether by virtue of alphabetic order, seniority or a simple twist of fate, Gen. Hays received her rank first, just a few minutes earlier than Hoisington. In a Pentagon ceremony attended by the wife of one of Gen. Hays's former patients, President Dwight D. Eisenhower, Westmoreland gave her the rank's silver star insignia and what Time magazine described as "a brassy kiss" on the lips.
It was, the Army chief of staff joked, all part of "a new protocol for congratulating lady generals."
The congratulatory peck has gone by the wayside, as dozens of women have since become general officers. Gen. Ann Dunwoody notably broke the "brass ceiling" in 2008 to become the military's first female four-star general.
At the time, however, Gen. Hays's promotion was greeted with astonishment from some quarters and derision from others. With good humor, she recalled receiving a letter from Germany addressed to the "Chief of the Feminine Army Sanitary Corps," and seeing a political cartoon that showed two men at a bar.
"Well, we've got everything, Sarge," the caption read, "the atomic bomb, guided missiles, the M16 rifle, and now two lady generals."
Anna Mae Violet McCabe was born in Buffalo on Feb. 16, 1920, and graduated from high school in Allentown, Pa. She received a nursing diploma in 1941 from Allentown General Hospital's School of Nursing and soon joined the Army Nurse Corps, inspired to serve after the Japanese attack on Pearl Harbor.
Gen. Hays, who took her last name from her husband William Hays, who died in 1962, performed much of her peacetime service at Walter Reed Army Medical Center in Washington, where she rose to became chief nurse in the emergency room. It was there that she met Eisenhower, who was hospitalized for an intestinal disease for about one month in 1956 and whom she considered a lifelong friend.
Gen. Hays received a bachelor's degree in nursing education from Columbia University's Teachers College in 1958, and graduated from the Catholic University of America in 1968 with a master's degree in nursing. Her military honors included the Distinguished Service Medal and the Legion of Merit.
A former resident of Arlington, Va., she leaves no immediate survivors, although friends sometimes encouraged her to start a family.
One day after she was promoted to general, she found herself at the hairdresser's next to Westmoreland's wife, Kitsy, Westmoreland later said. "I wish you'd get married again," Kitsy said. When Gen. Hays asked why, she replied: "I just want some man to know what it's like to be married to a general."
Read more Washington Post obituaries