If it seems as though everyone you know has been sick or has a child who is vomiting or has diarrhea (or both), that’s probably because of an increase in the number of acute gastroenteritis outbreaks in the region the past several weeks.

In Virginia, the number of outbreaks is about 20 percent higher than during the same period last year, health officials said.

Maryland health officials said that so far, the overall number of outbreaks in that state is comparable with recent years, but since January and February are peak times for the nasty and highly contagious bug, they are asking residents to take simple precautions, like washing their hands, to avoid infection.

The most common cause of acute gastroenteritis, or inflammation of the stomach and intestines, is a virus called norovirus. Symptoms can include vomiting, diarrhea, stomach pain, a low-grade fever, headache, muscle aches, chills and fatigue.

(Norovirus: How to stay healthy)

During peak season, outbreaks are common because the illness is easily spread, person-to-person, through contaminated food or water or by touching contaminated surfaces. People with noro­virus are contagious from the moment they begin feeling ill to at least three days — and perhaps as long as two weeks — after recovery, according to the U.S. Centers for Disease Control and Prevention.

Most people recover within a day or two.

The illness is especially common in schools, child-care centers, nursing homes, assisted living facilities and other closed environments where people are in close contact.

At Willow Springs Elementary School in Fairfax County, students began complaining last week of vomiting and diarrhea, and more than 100 students and several teachers were absent Friday. The school canceled all events Friday evening and over the weekend, and the building was thoroughly cleaned with a bleach solution, a spokeswoman for the county school system said. More than 100 students were absent Monday, but by Tuesday all but 16 had returned.

The illnesses have not been confirmed as caused by noro­virus, but doctors and health officials said it is the most likely culprit.

Earlier this month, about 85 students were infected by noro­virus at George Washington University. Last week, several had the bug at Howard University. In Montgomery County, health officials said that norovirus-like outbreaks have affected child-care facilities, nursing homes and schools.

In Maryland, there have been 87 outbreaks of norovirus-like illnesses, up from 58 last year. But the number of outbreaks is comparable to others within the past five years, health officials said. Generally, the state records an outbreak when during a seven-day period at least three people have the same symptoms.

In Virginia, there is no set number associated with the reporting of an outbreak, but it typically involves a group of cases with similar symptoms. There have been 120 outbreaks of suspected and confirmed norovirus since the beginning of the year. Although that number is higher than the 100 outbreaks reported for the same period last year, it’s “not shockingly more,” said Diane Woolard, a state epidemiologist.

“This is the time of year we expect it, and we’re seeing a lot of it,” she said. “I wouldn’t focus on the numbers. We just know it’s affecting people in the community.”

At Inova Loudoun Hospital, doctors in the pediatric emergency department have seen the number of cases involving vomiting and diarrhea without fever rise dramatically in the past two to three weeks. “We’re going from 20 to 30 a week to 50 to 60 a week,” said Ron Waldrop, who heads the department.

Doctors don’t typically run lab tests to check for norovirus if patients have the clinical symptoms, because the treatment is the same for all ordinary gastroenteritis cases. Once the more­serious causes of the symptoms are ruled out, such as appendicitis and other severe intestinal disorders, then “stomach virus is what we default to,” said Martin Brown, chairman of the emergency medicine department at Inova Alexandria Hospital.