Norovirus cases are on the rise in parts of the US, CDC data show

Cases of a nasty stomach bug are on the rise in some parts of the United States this winter, according to government data.

The latest numbers from the US Centers for Disease Control and Prevention show that 91 norovirus outbreaks were reported during the week of December 5, up from 69 outbreaks in the last week of November.

Numbers from recent years show a maximum of 65 explosions 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 crowded living situations such as nursing homes and prisons, as well as schools and places where people are in close proximity.

Here are some things you should 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 fewer than 10 viral particles capable of making someone sick, health experts say.

About 2,500 norovirus outbreaks are reported each year in the United States.

Eruptions can occur throughout the year, but are most common from November to April.

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

How do you get it?

Most norovirus outbreaks occur when people who are already infected spread the virus to others through direct means, such as through sharing food or eating utensils.

Outbreaks can also spread through food, water or contaminated surfaces.

How long are you sick?

Illness caused by norovirus usually begins suddenly, with symptoms developing 12 to 48 hours after exposure to the virus.

Most people get better within one to three days and make a full recovery.

But with 19 to 21 million illnesses each year in the United States, norovirus still causes an average of 900 deaths and 109,000 hospitalizations annually, mostly among adults age 65 and older.

It also leads to 465,000 emergency department visits, mostly involving young children.

Who is at risk?

People of all ages can be infected and become ill with norovirus.

Young children, the elderly and those with weakened immune systems are most at risk, with dehydration from vomiting and diarrhea the main concern.

There is no medicine 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 decreased urination, dry mouth and throat, and feeling light-headed when standing.

Dehydrated children may be extremely sleepy or restless and cry with little or no tears.

How can I protect myself?

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

Cleaning surfaces with household disinfectants can also help.