John Adams Elementary School in Alexandria will be closed Friday to contain the spread of a stomach illness that has affected a quarter of the students over the past two days, school officials announced Thursday.

Nearly 200 students and more than 30 staff members have reported symptoms of nausea, vomiting or diarrhea, and have stayed home or left school on Wednesday or Thursday.

City health officials said the most likely cause of the illness is norovirus, which is highly contagious and transmitted through contact with contaminated food or objects. There is no treatment, but symptoms usually improve in two days.

The school district has contracted with a cleaning service to disinfect all the surfaces in the school on Friday. Classes are expected to resume Monday.

Nancy Runton, acting health services coordinator for Alexandria schools, said it was an unusual move to close the school, but officials were concerned by the rapid spread of the illness.

On Wednesday night, 33 staff members called in sick, and five more went home during the day Thursday. Staff went room to room to check attendance Thursday and found that 160 children were out, a dramatic increase from the previous day. Dozens more left during the day.

“It was a constant calling of parents during the school day to come and pick up more kids,” she said.

Norovirus is so contagious in part, Runton said, because it can live on surfaces for up to two weeks.

The city’s Health Department recommends that children wash their hands frequently with soap and water for at least 20 seconds, and is asking parents to keep their children home from school if they develop symptoms of vomiting or diarrhea until 24 hours after the symptoms have stopped.

“We know it causes a lot of disruption” to cancel school, she said. “We hope that getting ahead and being proactive quickly will stem the tide.”