LITITZ, Pa. (AP) — Mary Read had been a nurse for years, using her extensive training to care for patients in nursing homes and doctors' offices.

"But all of a sudden I was forgetting stuff," she says. "I didn't understand what the doctor wanted me to do — and it was stuff I'd been doing and suddenly couldn't do."

She had trouble calculating a patient's height from inches to feet, for example, or she would forget how to spell the doctor's name.

It was stress, they thought. Maybe a virus attacking her brain.

"I was fired from my job," she says. "I knew something was wrong."

She went to her fami...