Some places in the U.S. are seeing an increase in cases of gastrointestinal illnesses known as noroviruses.

According to the U.S. Centers for Disease Control and Prevention, 91 outbreaks were reported the week of Dec. 5, up from 69 cases the last week of November, according to the Associated Press. In recent years, a maximum of 65 outbreaks have been reported in the first week of December.

The highly contagious infection is the most common cause of foodborne illness and causes sudden vomiting and diarrhea. Other symptoms include nausea, stomach pain, body aches, headaches and fever. There are no medications for treatment.

According to the CDC, there are between 19 and 21 million norovirus cases each year, and an estimated 900 people die from it while 109,000 are hospitalized. It is estimated that one in 15 people are infected each year.

Noroviruses are commonly found on cruise ships, nursing homes, prisons, schools and in crowded living conditions. In the United States, cases typically peak between November and April

Experts say masking doesn’t do much to prevent norovirus because it is spread through direct contact with an infected person, surface, or food and water — rather than through the air.

They recommend washing hands with soap and water, drinking clean water and following food safety regulations. Infected people usually feel better after about one to three days.