Until 2010, Amelia Nuwer, 22, visited the same dentist every year in Biloxi, Miss., her hometown. And every year she came back with a clean bill of dental health: no fillings necessary.

Then, as a junior at the University of Alabama, she saw a new dentist who delivered her first negative diagnosis: two cavities. Six months later, the dentist told her she had two more. Earlier this year, he once again had bad news: yet another cavity.

Somehow, in 12 months she had gone from perfect oral health to five fillings. “It felt wrong to me,” she said.

Her hometown dentist, Dr. Francis Janus, was surprised, too. He examined his longtime patient after she graduated. Ms. Nuwer’s so-called cavities, he concluded, had actually been “incipient carious lesions,” a form of early-stage decay that some dentists call “microcavities.”