A stomach illness suspected to be the highly contagious norovirus has sickened at least 100 people who took part in a regional dance competition last weekend, health officials said Wednesday. Norovirus, also called the Norwalk virus, causes nausea, vomiting, diarrhea and stomach cramping, and can lead to more serious illnesses in the elderly and infirm. Billings suffered a community-wide outbreak last year, and several nursing homes were struck with the disease earlier this winter. “It’s still early in our investigation. We don’t know exactly what we’re dealing with yet,” said Barbara Schneeman, spokeswoman for the Yellowstone City-County Health Department. Schneeman said most of those reporting symptoms took part in or attended the Spotlight Dance Cup Regional Dance, held March 9-11 at the Billings Holiday Inn.