The Kalamazoo County health department has issued an advisory about a “sharp increase in the number of people ill with norovirus” in the past week.

“We don’t have any numbers” because norovirus cases don’t have to be reported to the health department, said Linda Buzas, Kalamazoo County health director. But “we’re getting lots of calls from lots of different places” reporting a spike.

Norovirus causes nausea, vomiting and/or diarrhea, abdominal cramps and sometimes a low-grade fever. The symptoms last 12 to 48 hours and will go away without treatment.

“More than 90 percent of what people call stomach flu is norovirus,” Buzas said. “It comes on very quickly and leaves quick, but it’s very irritating and will make you very miserable.”

Norovirus is “extremely contagious” — thus, the point of the health alert, Buzas said.

“It spreads very easily, and the big problem is that people don’t stay home long enough,” further spreading the disease, she said.

People with the illness are contagious up to 48 hours after their symptoms subside, and they should stay home from work, school and public gatherings during that time.

Also, the spread of norovirus can be prevented through “excellent hand washing,” the health advisory said.

“It’s really important for people to wash their hands” during a norovirus outbreak, Buzas said.