Lara Thompson was 26 when her life fell apart.

She was living in Rhode Island and working in HIV prevention research when she unexpectedly developed nausea and diarrhea. It was early 2008, a few weeks after New Year’s, and she thought she might have picked up a stomach virus at a holiday gathering, or stressed her system with overindulgence. She expected the symptoms would pass after a few days. They didn’t.

“In three weeks, I dropped 15-20 pounds,” she says now. “I couldn’t keep anything in; I would have to run to the bathroom at a moment’s notice. I was so lethargic I had to stay home from wor...