Know someone with a bad stomach bug this holiday season? They're not alone -- cases of norovirus are surging in parts of the United States this winter, according to data from to the Centers for Disease Control and Prevention.
The most recent numbers from the CDC 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.
Norovirus outbreaks occur throughout the year, the CDC said, but are most common November through April. Outbreaks typically occur when an infected person spreads the virus through direct contact, like sharing eating utensils or food at a group gathering, holiday party or restaurant.
So far this month, "several small outbreaks" of suspected norovirus cases were reported to the Chicago Department of Public Health, a spokesperson said.
"This is not out of the ordinary for this time of year," the spokesperson added.
But what exactly is norovirus, and what are the symptoms? Here's what to know.
What is norovirus?
Norovirus -- also referred to as "the stomach bug" or "food poisoning" -- 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.
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According to the CDC, norovirus is the leading cause of foodborne illness in the United States, responsible for 58% of such infections acquired in the country each year.
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.
Norovirus symptoms
Along with vomiting and diarrhea, common symptoms include nausea, stomach pain, body ache, headache and fever.
How does norovirus spread?
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 does Norovirus last?
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 to protect yourself from norovirus
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.
