If you're experiencing nausea, stomach pain, vomiting and diarrhea, you may have a nasty stomach bug known as norovirus.

Here's what you need to know about norovirus, which can affect mostly anyone, ranging from Olympic athletes to a group of beachgoers in Maine.

What is norovirus?

Norovirus is also referred to as the “winter vomiting bug," Lee-Ann Jaykus, the scientific director for NoroCORE, a food safety initiative that’s funded through a $25 million grant from the USDA, told Fox News.

The virus is not uncommon, as roughly 21 million Americans get the virus each year, according to the CDC.

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Norovirus enters the body through the mouth, reaches a person's gastrointestinal tract and inflames the stomach or intestines, or both. As a result, it causes nausea, stomach pain, vomiting and diarrhea, which leads to dehydration. It can also cause fever, headaches and body aches.

While the symptoms can be severe, most people recover within days.

How does it spread?

Norovirus, which has different strains, can spread easily —especially in closed spaces, Jaykus said.

A common way the virus is transmitted is through close contact with an infected person. This is either directly or indirectly; such as sharing a bathroom, a dorm room or another communal space. Cruise ships, schools and nursing homes “are the most commonly reported settings for norovirus outbreaks,” according to the CDC.

More specifically, however, the virus spreads through fecal matter and vomit.

When an infected person vomits or defecates, “massive amounts [of the virus] are excreted,” Jaykus explained. “There are millions to billions of particles in one just one gram.”

“It only takes a few virus particles to make people sick,” she added.

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The virus also spreads through food and contaminated water.

Food handlers who are sick and don’t practice adequate hygiene can easily infect other people. Infected water can also spread the illness, though this more commonly occurs in developing countries, Jaykus noted.

How do you prevent it?

Unlike the flu and other illnesses, there is no vaccine to prevent norovirus.

Keeping your hands washed and thoroughly cleaning contaminated surfaces is key to prevent the spread of infection.

“Hand washing is hugely important,” Jaykus said. “In fact, it’s the single most important thing for people attending Olympics,” she said.

WHY ARE CRUISE SHIPS SUCH HOTBEDS FOR THE NOROVIRUS?

Norovirus is extremely hard to kill, Jaykus warned. Inactivating the virus requires a high concentration of bleach. And while it’s easy enough to clean countertops and other similar surfaces with bleach, the same can’t be said for carpet and furniture. Alcohol isn’t strong enough to entirely kill the virus either, Jaykus added.

“Norovirus can be spread for weeks,” said Jaykus, who added that quarantining infected people can also be useful in preventing the spread of norovirus.

The Associated Press contributed to this report.