Montgomery County health officials activated their emergency alert system Tuesday afternoon to warn residents of an early outbreak of norovirus, a highly contagious flu-like disease that has infected between 75 and 80 residents in the past month. The county typically reports about a dozen cases of the illness, which tends to occur in January or February.

The intestinal virus or “cruise ship illness” is common in places where large groups of people live closely together.

Mary Anderson, a spokeswoman for the county’s health department, said outbreaks have been reported at two nursing homes. She acknowledged Gaithersburg’s Wilson Health Care Center at Asbury Methodist Village was affected, because the outbreak was reported by the Gazette newspaper, but said confidentiality rules prevented her from identifying the second.

A group of six or seven county residents who attended an out-of-state wedding also contracted the disease, as did a small group attending a business luncheon, Anderson said.

Norovirus is spread through the air when people vomit and through things such as not washing one’s hands after using the toilet or changing a diaper and then touching food and other surfaces such as doorknobs, light switches and handrails. People may only feel sick for a day or two, but can continue to spread the virus for as long as a week or more after they begin to feel better.