School nurses' offices and hospital emergency rooms have been filling up with kids and adults stricken with quick-hitting gastrointestinal viruses.

The virus peaked about two weeks ago at Marston Elementary in Hampton, but it hasn't been wiped out completely. The school had 20 third-graders out Monday, according to school nurse Kathy Connolly.

Connolly said the symptoms have been mostly upset stomach and vomiting with no fever.

“Generally it hits and it's quick and you're feeling better in two days,” she said.

The emergency room at Manchester's Elliot Hospital has seen more adults and children coming through the doors in recent weeks suffering from gastrointestinal illnesses, some with severe cases involving prolonged vomiting and diarrhea, according to Dr. Richard Marcucci, the hospital's medical director for emergency services.

Some patients have been admitted and given intravenous fluids because they were so dehydrated, Marcucci said.

“We've seen a fair amount of it. It's been a slow and gradual uptick since the holidays,” he said. “This time of year you see a bit more of it, but it's atypical in a sense that we've seen a bit more volume. I wouldn't call it an outbreak, but there was definitely a larger number of severely ill patients with gastrointestinal (illnesses) than we would normally see this time of year.”

Marcucci said some patients were infected with noroviruses, which causes acute gastroenteritis and usually involve diarrhea, vomiting, and stomach pain.

People with norovirus are contagious from the moment they begin feeling ill to at least three days and perhaps for as long as two weeks after recovery. Noroviruses are found in the stool and vomit of infected people.

Diagnosis is often based on the combination of symptoms and the short duration of the illness; most people get better within one to two days.

Chris Adamski, state Department of Health and Human Services bureau chief for infectious disease control, said in January that the state typically has 40 to 80 outbreaks of gastrointestinal illness a year. Between Dec. 1 and mid-January, the state investigated 16 reports of outbreaks, most of which occurred in health care facilities, such as nursing homes, hospitals or assisted-living centers.

Things haven't been too bad at Underhill School in Hooksett. Principal Andrew Bairstow said the school has seen some students with stomach aches but there's been no significant outbreak. Still, the school has taken steps to educate students about proper handwashing by having the school nurse visit classrooms and also addressing it at a schoolwide assembly.

“We're trying to cut things off at the pass,” Bairstow said.

The story was similar at Londonderry North School.

“It seems to be the usual. It's been a little bit of this and a little bit of that,” said Anna Cook, school nurse.

Smyth Road Elementary School in Manchester saw an increase in illness over the past month, with an average of two kids being sent home a day. Officials said symptoms were mostly vomiting.

New Hampshire Union Leader Staff Writer Kathy Marchocki contributed to this report.