PORTLAND – Feeling a little under the weather? You’re not alone.

The Maine Bureau of Health has been fielding calls about people who are suffering from nausea, vomiting, diarrhea and abdominal cramps as the state sees an apparent upsurge in Norwalk-like viruses.

“This seems to be a good Norwalk season,” said Dr. Kathleen Gensheimer, the state epidemiologist. “We have certainly had reports of clusters in nursing homes and schools and community types of settings.”

It’s not totally clear whether there’s an actual increase in the number of cases or whether people are simply more aware of the illness, which has been splashed across front pages because of large outbreaks on cruise ships.

Most people had never heard of it before cruise ship passengers started getting sick, but Norwalk-like viruses are quite common.

They are responsible for at least half of Maine’s outbreaks of food-borne illness. Nationally, Norwalk viruses strike 23 million Americans each year, and they are responsible for 50,000 hospitalizations and 300 deaths. Most people recover within two or three days.

Laurel Brady, a spokeswoman for Mercy Hospital, said the Portland hospital’s emergency department has seen “a definite increase” in Norwalk-like gastrointestinal problems.

“It’s kind of been the name of the game around there lately,” she said. “They’ve had over the past two weeks lots of people come in with nausea, vomiting, diarrhea. It varies day to day.”

During the past week, doctors in Maine Medical Center’s emergency department in Portland have been caring for as many as five to 10 people a day with acute gastrointestinal symptoms, said Martha Davoli, a hospital spokesperson.

“We don’t know if it’s the Norwalk virus because they just treat it symptomatically and try to help people feel better,” she said. “But it is definitely more people than usual who are presenting with these symptoms.”

The state does not keep track of the number of cases because the virus is not diagnosed in the laboratory, and many people get over their illnesses without seeing a physician at all.

So there’s no way to compare this year’s spate of illnesses with any other year. But when outbreaks occur at nursing homes, day care centers and similar facilities, Gensheimer’s office usually gets a call. A Norwalk diagnosis is usually made by excluding other causes of the symptoms, she said.

Norwalk-like viruses spread easily for several reasons.

“They’re extremely contagious because of their low infectious dose, which is considered to be less than 100 viral particles,” Gensheimer said.

The Norwalk virus is also difficult to kill. It is highly resistant to disinfectant, and remains stable at both freezing and 140 degrees.

People can remain contagious up to two weeks after they recover, which is another reason hand washing is so important.

“It’s a virus that’s easily disseminated in the environment, it seems to persist very well, and you really need to be very careful with the same message that I always preach: hand washing, hand washing and hand washing,” she said.