SAN JOSE — Santa Clara County public health officials on Tuesday confirmed an outbreak of viral gastroenteritis in at least a dozen schools around the county, forcing some schools to cancel all non-instructional events this week — and possibly beyond.

The outbreak follows outbreaks of a similar illness called norovirus that have occurred recently around the state, including in Northern California’s Yolo County, where at least 1,000 students, teachers and staff were sickened by norovirus this month. Since January, San Mateo County has reported only a handful of norovirus cases.

Around 200 students in nine San Jose Unified School District elementary schools and one middle school have come down with one of the symptoms of viral gastroenteritis — which attacks a person’s intestines and often causes nausea, vomiting or diarrhea — since early May, said Melinda Landau, the school district’s manager for health and family support programs.

She said the first cases surfaced at Hacienda Environmental Science Magnet School around May 4 after parents of at least two children from the same class called to say their children were sick and would not be attending class.

As the cases multiplied at Hacienda — which had reported 80 cases as of Tuesday — and at other schools, officials have been notifying parents with letters, emails and phone calls.

Landau said Anne Darling and Horace Mann Elementary schools each have reported 27 cases; Olinder Elementary has 26; Washington Elementary has 11; Grant Elementary has 10; Terrell Elementary has 9; Willow Glen Elementary has 8, Carson Elementary has 4 and John Muir Middle School has 3.

Landau said all “high-touch” surfaces at any school where the stomach flu has been reported are being cleaned with a bleach solution — including desks, chairs, counter tops, door handles, light switches, cafeteria furniture, playground equipment and bathrooms.

Dr. George Han, Santa Clara County’s deputy public health director, said the illness is not uncommon, but that the number of schools and children currently affected since the first reports in late April is “higher than usual.”

He said at least 12 schools around the county have reported incidents to the health department, which in turn must relay that information to the state public health department.

Still, it’s hard to tell what’s behind the outbreak.

“Unfortunately, it’s very hard to predict,” said Han. “But the good news is it’s a relatively mild illness — you feel really bad for a day or two.”

Han said the message public officials want transmitted is that “parents should keep their kids at home when they get sick — and for 48 hours until after they get better,” he said.

That’s because even though the symptoms have disappeared, children can continue to shed the virus for at least two days afterward.

“It’s why it’s so difficult to control because kids feel better and they’re up and running around, but their body is still shedding the virus” through their stool. And if they don’t wash their hands thoroughly after going to the bathroom, the potential for passing it on to other people continues.

Han said now is also a good time for parents to review with their children the proper way to wash their hands with soap and water, including using the ABC rule: Say the entire alphabet while washing your hands, and don’t stop until you get to the letter Z.

Landau said the timing is curious because it’s occurring toward the end of the school year, just as it did last year when a wave of stomach flu sickened dozens of students at San Jose’s Horace Mann Elementary School and other district schools.

“It’s a sad time because we’re having plays and other events we have to be thinking about canceling because we don’t want to put people who are potentially sick in the same room and have them spread it to other people who aren’t sick,” she said.

At Hacienda, for example, Open House, Art Night, the Book Fair and the Principal’s Coffee scheduled for this week have all been canceled voluntarily as a precautionary measure.

HOW TO AVOID STOMACH FLU:

Here are some tips from the U.S. Centers for Disease Control and Prevention to help protect yourself and others from getting the stomach flu:

Wash your hands carefully with soap and water, especially after using the toilet and changing diapers, and always before eating, preparing or handling food.

When you are sick, do not prepare food or care for others.

Clean and disinfect contaminated surfaces.

Wash fruits and vegetables, and cook seafood thoroughly.

Wash laundry thoroughly.