Among the 400 women who served on both sides of the Civil War, Sarah Emma Edmonds enlisted as a nurse. A male nurse.
    Her deception had a certain ironic twist, given her father's disappointment at her birth. He freely expressed his resentment at having a daughter, but when he demanded that she marry a man he had selected, Edmonds escaped her native Nova Scotia and eventually settled in Flint. Enamoured with her new country, she longed to serve when the call came and failed three times before successfully enlisting in the Union Army as "Frank Thompson." (At the time, the Army physical consisted of answering a series of questions.)
    Edmond's outfit, the 2nd Michigan Volunteers, was sent to support Union Army leader George McClellan's campaign in Virginia, and because of the primitive conditions, she had little trouble concealing her true identity. When an intelligence post opened, "Private Thompson" volunteered and arrived for her interview so well-prepared, she was chosen for the assignment.
   She infiltrated the Confederate Army as a Black man, an Irish peddler, a Black washerwoman and a young Confederate sympathizer, all the while gathering valuable information used by Union leaders. While her unit served with General Grant in the Battle of Vicksburg, Edmonds contracted malaria and, fearing her true identity would be discovered, ran to a civilian hospital for treatment.
    With "Private Thompson" branded as a deserter, Edmonds couldn't return to her former position once she recovered. She traveled instead to Washington, where she continued to work as a female nurse until the end of the war. She eventually returned to Canada, married and had three sons, one of whom enlisted in the Army.
    Edmonds wrote a book about her experiences, titled Unsexed or the Female Soldier and later re-titled Nurse and Spy in the Union Army. She also petitioned for and received an honorable discharge and her Army pension, after relating to Congress the details of her military service.

Sources:

Michigan Women's Hall of Fame
Civilwarhome.com
Exceprt from Nurse and Spy in the Union Army
Archives of Michigan - Sarah Emma Edmonds
US Dept. of Energy Hanford Site-Spy Briefing Center