Cases of gastrointestinal bacteria are becoming more common in parts of the United States this winter, according to government data.

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

Figures from recent years show that a maximum of 65 outbreaks were reported in this first week of December.

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Norovirus infection is characterized by sudden vomiting and diarrhea. Outbreaks often occur on cruise ships, in congregate settings such as nursing homes and prisons, as well as in schools and places where people live close to one another.

Here are some things you should know about the virus.

What is Norovirus?

According to the CDC, norovirus is the leading cause of foodborne illness in the United States, accounting for 58 percent of infections occurring annually in the country.

Norovirus infections are caused by a group of viruses that spread easily, with just 10 virus particles capable of making someone sick, health experts say.

Approximately 2,500 norovirus outbreaks are reported annually in the United States. Outbreaks can occur any time of the year, but are most common between November and April.

In addition to vomiting and diarrhea, common symptoms include nausea, stomach and body pain, headaches and fever.

How do you get it?

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

How long will you stay sick?

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

But even though there are 19 to 21 million cases of illness each year in the United States, norovirus still causes an average of 900 deaths and 109,000 hospitalizations per year, primarily in adults age 65 and older. It also results in 465,000 emergency room visits, mostly involving young children.

Who is at risk?

People of all ages can become infected and become ill with norovirus. Young children, the elderly and people with weakened immune systems are most at risk, with dehydration from vomiting and diarrhea the biggest concern.

There are no medications to treat norovirus. To rehydrate, we recommend drinking water and other liquids, with the exception of coffee, tea and alcohol.

Anyone suffering from dehydration should seek medical attention. Symptoms of dehydration include decreased urine output, dry mouth and throat, and dizziness when standing. Dehydrated children may be unusually sleepy or fussy and cry little or not at all.

How can I protect myself?

Thorough and frequent hand washing is the best protection against norovirus during the peak winter season. Wash hands with soap and warm water for 20 seconds before meals.

Scrubbing surfaces with household disinfectants can also help.