Sister Margaret started life as just plain Margaret, no last name. She was a foundling and was raised in a Catholic orphanage by the Sisters of Mercy, a Franciscan order. Early in life, she heard the call to take vows and started her novitiate at age 11. She became a teacher and, in time, started work at the orphanage herself. Sister has led a rather sheltered life, leaving the grounds only for field trips and to interview prospective parents for her charges.
Then, early in the summer, the children started getting sick. Then the other sisters started getting sick. Then they were all dead, and only Sister Margaret left behind with too many bodies to bury and sick at heart.
She left the orphanage for the last time and started moving west, following a strange call that spoke in her dreams in the voice of an old black woman.