Santa Clara County health officials confirmed Wednesday that more than 350 South Bay students have contracted norovirus, adding to the thousands of cases reported at other Northern California schools this week.

Of the 13 San Jose schools that have been affected, Hacienda Environmental Science Magnet School seems to have been hit hardest, with between 80 and 90 students having called in sick so far.

No classes have been canceled, but the San Jose Unified School District has stopped all extracurricular activities, including after-school programs, this week. It remains unclear when the programs will resume.

District officials say they notified the Santa Clara County Public Health Department after the first case was reported on May 4. The department hasn't confirmed norovirus through lab testing, but officials are confident in their diagnosis since the children have specific symptoms of the viral gastroenteritis.

The most common norovirus symptoms include nausea, diarrhea, vomiting and stomach pain, according to the Centers for Disease Control and Prevention. The virus spreads through contact, such as eating food after an infected person, touching a contaminated surface and then putting your fingers in your mouth, or having direct contact with an infected person.

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 nine; Willow Glen Elementary has eight; Carson Elementary has four; John Muir Middle School has three; and Empire Gardens Elementary has an unknown number.

Health officials put the total number of affected San Jose students at 242.

Two other schools outside the San Jose Unified School District also were affected by norovirus, the health department said: Rosemary School in Campbell and Jackson Elementary School in Morgan Hill. Those two schools reported a combined 129 sick students, health officials said, the vast majority of which occurred in April.

The cases reported in Santa Clara County add to the cases reported in nearby Yolo County. In a Friday statement, the county said 952 students, teachers and staff from 32 schools contracted the virus. The statement said the number of sick people was "increasing every day at a very alarming rate."

A Yolo County spokeswoman then said Wednesday the number of sick cases had risen to at least 2,091, NBC affiliate KCRA reported.

Crews in San Jose on Wednesday used bleach to disinfect desks, chairs and countertops at several schools. Playground equipment, some of which is still off-limits, is also being wiped down. Officials in Yolo County advised that the same precautions be taken there.

Nurses went to Hacienda to teach students how to properly wash their hands.

"We really encourage parents, if their kids are experiencing symptoms, to keep them home 48 hours symptom-free after they've stopped showing symptoms without medication that's sort of the baseline we're using on when it's safe to send their kids back to school,” said Peter Allen with the San Jose Unified School District.

