Classes in an entire school district in Colorado have been canceled Thursday and Friday after a suspected outbreak of the highly contagious norovirus.

In recent days, hundreds of students and staff in Mesa County Valley School District 51 have been complaining of an illness that comes on “incredibly fast” and leads to vomiting and diarrhea.

The problems started last week when about 30% of students and 20% of staff called in sick at a high school, said Emily Shockley, a spokeswoman for the district.

The outbreak seemed to be contained just to that one high school, which has about 1,050 students, until this week, when other schools began to be affected.

“Then it was kind of a domino effect,” Shockley told NBC News. “It got to be too much so we decided to close the schools.”

The exact illness remains unknown, but it’s being treated as a norovirus outbreak, even without lab confirmation, based on the symptoms, Mesa County Public Health said in a statement.

“We’re hearing reports that a person might feel just fine, and in less than an hour’s time have a vomiting episode,” the agency noted.

Viral and gastrointestinal illnesses are very common and the current uptick in cases is in line with what health officials typically see at this time of year when bugs begin to spread, it added.

Still, the school district, which has about 22,000 students, has never before had to close all schools due to illness.

“We are taking this highly unusual action because this virus is extremely contagious and spreading quickly across our schools," said Tanya Marvin, the district’s nursing coordinator, in a statement.

"In addition, it appears that there is now a second, related virus that is affecting students, some of whom have already been ill in recent weeks. The combination of the two has created an unprecedented spread of illness.”

Closing all of the schools now will allow cleaning crews to disinfect the buildings, added Superintendent Diana Sirko. The decision, ahead of the Thanksgiving break, means students won’t be back in class until at least Dec. 2.

'One of the most infectious viruses'

This time of year marks the beginning of the norovirus season so it’s common to see such outbreaks in the winter months, said Charles Gerba, a microbiologist and professor of public health at the University of Arizona.

“Noroviruses are one of the most infectious viruses known,” Gerba told NBC News. “Usually, up to 70% of the people have projectile vomiting — which is loaded with the virus, which helps it spread.”

Patients with the illness can shed billions of norovirus particles, only a few of which can make other people sick, according to the Centers for Disease Control and Prevention. Outbreaks are very common, especially in healthcare facilities, restaurants, schools and on cruise ships. One person can infect hundreds of others.

Norovirus is sometimes called the stomach flu or stomach bug, but it's not related to the flu. There's no real treatment for it except for waiting it out.

People can get infected by having direct contact with an infected person, consuming contaminated food or water, or touching contaminated surfaces and then putting an unwashed hand in their mouth.

There’s no vaccine to prevent norovirus, so the best way to avoid getting sick is to wash your hands with soap and water routinely throughout the day and especially after going to the bathroom, before handling food, and after contact with doorknobs, keyboards, light switches, handrails and other surfaces frequently touched by other people.

When it comes to schools, it’s a good idea for kids to use disinfectant wipes on their desk tops because that’s where most of the noroviruses in classrooms are found, Gerba said. Other school "norovirus hot spots" include the toggle on the water fountain and the pencil sharpener.