The most recent numbers from the U.S. Centers for Disease Control and Prevention show there were 91 outbreaks of norovirus reported during the week of Dec. 5, up from 69 outbreaks the last week of November.

Numbers from the past few years show a maximum of 65 outbreaks reported during that first week of December.

A norovirus infection is characterized by sudden vomiting and diarrhea. Outbreaks are often seen on cruise ships, in congregate living situations like nursing homes and jails, as well as schools and places where people are close together.

Here are a few things to know about the virus.

What is norovirus?

Norovirus is the leading cause of foodborne illness in the United States, responsible for 58% of such infections acquired in the country each year, according to the CDC.

Norovirus infections are caused by a group of viruses that spread easily, with as few as 10 viral particles having the ability to make someone sick, health experts say.

There are about 2,500 norovirus outbreaks reported annually in the United States. The outbreaks can occur throughout the year but are most common from November to April.

Along with with vomiting and diarrhea, common symptoms include nausea, stomach pain, body ache, headache and fever.

How do you get it?

Most norovirus outbreaks occur when people who are already infected spread the virus to others by direct means, such as through sharing food or eating utensils. Outbreaks can also be spread through food, water or contaminated surfaces .

How long do you stay sick?

Illness caused by norovirus typically starts suddenly, with symptoms developing 12 to 48 hours following exposure to the virus. Most people get better within one to three days and recover fully.

But with 19 to 21 million illnesses each year in the United States, norovirus nevertheless causes on average 900 deaths and 109,000 hospitalizations annually, mostly among adults aged 65 and older. It also leads to 465,000 emergency department visits, mostly involving young children.

Who’s at risk?

People of all ages can get infected and fall sick from norovirus. Young children, older people and those with weakened immune systems are most at risk, with dehydration from vomiting and diarrhea the top concern.

There is no medication to treat norovirus. Rehydration is recommended by drinking water and other liquids, with the exception of coffee, tea and alcohol.

Anyone suffering from dehydration should seek medical help. Symptoms of dehydration include a decrease in urination, dry mouth and throat, and feeling dizzy when standing. Dehydrated children may be unusually sleepy or fussy and cry with few or no tears.

How can I protect myself?

Rigorous and frequent handwashing is the best defense against norovirus during the peak winter season, scrubbing the hands with soap and warm water for 20 seconds before meals.

Scrubbing surfaces with household disinfectants can also help.