When the small spot first appeared on her nipple, Karrie Morgan didn’t think much of it. She was shocked nearly a year later when she learned it was a symptom of Paget disease, a rare type of breast cancer that affects 4% of patients.

Over the course of that year, the spot had grown. When Karrie began covering it with a bandage, she started to suspect that something wasn’t right. Then, during a routine exam, her gynecologist recommended...