It’s that time of the year again: Black Friday, Thanksgiving, influenza, and norovirus. The winter cold months are the perfect environment for some pathogens to spread.

Norovirus, also called the “winter vomiting bug”, is a very contagious virus that causes vomiting and diarrhea. Norovirus is spread by coughing, sneezing and close contact, or touching the same surfaces. People infected with norovirus can shed billions of norovirus particles and only a few virus particles can make other people sick.

About 60 schools in north-east England have been hit by a suspected outbreak of norovirus. Some schools in the region had to close down last week and undergo a ‘deep clean’, after hundreds of staff and pupils were hit with vomiting, diarrhea and flu-like symptoms.

Public Health England (PHE) said it was not able to give an exact figure of the number of schools that have been affected, nor their location. However, figures published by PHE suggested norovirus rates are 26 percent higher than they usually are at this time of year.

Between October 28 and November 10, a total of 332 people were infected by the highly contagious bug. A total of 18 outbreaks caused hospital wards to close or to restrict admissions across England and Wales. PHE said it expects these types of bugs to go around schools and workplaces during this time of year, as norovirus is predominantly a ‘winter pathogen’.

On the other side of the Atlantic, Mesa County Valley School District 51 – a school district in Colorado, United States – announced the closing of the entire school district through the end of this school week. All 46 schools in the district reported several students and teachers have gotten sick with vomiting and diarrhea. Although the cause has not yet been clearly identified yet, experts believe this is most likely norovirus.

Noroviruses are thought to be the most common cause of acute gastroenteritis (diarrhea and vomiting). On average, noroviruses cause 19 to 21 million cases of acute gastroenteritis, 1.7–1.9 million outpatient visits and 400,000 emergency department visits in the U.S. per year, according to the CDC.

Young children, the elderly, and people who have a weakened immune system are particularly susceptible to catching noroviruses. The spread of the virus can be hard to control because it’s contagious before symptoms appear.

The Think Noro public health campaign advises:

N – “No visits to hospitals, care homes and GP surgeries if you are suffering from symptoms of Norovirus – send someone else to visit loved ones until you are better.”

O – “Once you’ve been symptom-free for at least 48 hours, you’re safe to return to work, school or visit hospitals and care home.”

R – “Regularly wash your hands with soap and warm water, especially after using the toilet, and before eating or preparing food.”

O – “Only hand-washing will prevent the spread of Norovirus – alcohol hand gels DON’T kill the virus.” Hand sanitizers are not effective against norovirus; soap is your best weapon.

There is still no licensed vaccine against norovirus, but there are promising candidates in the pipeline.