After almost 90 years of controversy, Sister Mug's miraculous healing from cancer was finally accepted in 2006 by the Holy See as Guérin's first miracle for sainthood.

Mug suffered from neuritis, a more painful form of neuropathy, that inhibited her ability to carry out her teaching duties at St. Mary-of-the-Woods college.

On October 30, 1908, sister Mug prayed at the Saint-Mary-of-the-Woods motherhouse, kneeling before Guérin's crypt in the Church of the Immaculate Conception.

She didn't pray for herself (although she could have, given her health), but for another sister, Joseph Therese O'Connell, who was also ill. "While standing there," she wrote to Mother Mary Cleophas, "the thought came into my mind, 'I wonder if she has any power with Almighty God.'

She had suffered nerve damage following a mastectomy for breast cancer at the hospital in Terre Haute, and she had pain in both arms and her right hand, which were difficult to use.

This knowledge made her a natural choice to do the writing when Silas Chatard, Bishop of Indianapolis, asked the congregation for a book about Mother Guérin.