(NewsNation) — On par with seasonal trends, norovirus cases are on the rise across the country, according to the U.S. Centers for Disease Control and Prevention. However, positive testing rates are lower compared to this time last year.

Still, norovirus is most common during this time of year, and health officials are warning Americans to be aware of outbreaks in their area.

Last month, people in Southern California were warned to avoid eating raw oysters sourced from parts of Mexico after more than 200 people recently fell ill with suspected cases of norovirus.

What is norovirus?

Norovirus is a very contagious illness that may also be referred to as viral gastroenteritis, stomach flu or food poisoning. However, the CDC says the common virus is not related to the flu. It’s also the most common cause of vomiting, diarrhea and foodborne illnesses, according to the agency.

What are norovirus symptoms?

Symptoms usually develop within 12 to 48 hours after being exposed to the virus. Those symptoms include diarrhea, vomiting, nausea and stomach pain.

Other symptoms can include fever, headache and body aches. The virus can cause inflammation in the stomach and intestines, called gastroenteritis, according to the CDC.

The good news: symptoms typically end within two days, the CDC says. However, children, older adults and immunocompromised people have an increased risk of dehydration.

It’s important to note symptoms of dehydration include a decrease in urination, dry mouth and throat and feeling dizzy when standing up.

How is norovirus spread?

While the virus is most commonly spread in schools, day care centers and nursing homes, health officials say anyone can contract it.

“People can become infected by eating food or drinking liquids that are contaminated with norovirus, touching surfaces or objects that are contaminated with norovirus, and then placing their hand in their mouth, having direct contact with an infected person,” according to the Rhode Island Health Department.

Having direct contact with someone with the virus is the most common way to contract the illness, especially when sharing food or eating utensils, according to the CDC. It can also spread through contaminated food, water and surfaces.

“If you get norovirus illness, you can shed billions of norovirus particles that you can’t see without a microscope. It only takes a few norovirus particles to make you and other people sick,” the CDC says.

A person who has contracted norovirus is most contagious while experiencing symptoms and during the first few days of recovery.

Where is norovirus spreading?

While cases are up nationally, cases are highest in the Northeast region. According to the most recent CDC data, more than 13% of norovirus tests came back positive. Western states are also seeing about a 12% positive rate in tests. Southern and Midwestern states hover around a 10% positive test rate.

How is it preventable?

Here are some tips to prevent contracting norovirus:

• Use proper hygiene. Wash your hands with soap and water, especially after using the bathroom and before/after eating. An addition (not a substitute) of alcohol-based hand sanitizer can also help combat harmful bacteria.
• Be careful in the kitchen. Wash fruits and vegetables before cooking or eating them. Cook any shellfish before eating them.
• Limit activity if you’re infected. Do not prepare food for others while you have norovirus symptoms to avoid spread.
• Disinfect. After a sick episode, clean any potentially contaminated surfaces with a bleach-based cleaner.
• Wash laundry. Any clothing that may be contaminated should be washed immediately to limit the spread of the virus.

If you or someone you know has contracted norovirus, the CDC recommends drinking a lot of liquids to replace the fluids lost from being ill. If symptoms persist, or someone suffers from severe dehydration, contact a doctor immediately.

NewsNation affiliate WPRI and the Associated Press contributed to this report.