Cases of norovirus are rising across the United States, doubling over the last few months, according to updated data from the Centers for Disease Control and Prevention (CDC).

Nearly 14% of tests came back positive for norovirus during the most recent week ending Nov. 15, compared to roughly 7% about three months ago.

Test positivity is a metric used to identify how many people are confirmed to have norovirus after an illness is suspected.

Health experts have said that, like other measurements, test positivity is imperfect as it depends on the number of tests that are ordered, which can vary.

Last December saw a record-breaking rate of norovirus cases in the U.S. with nearly 25% of tests coming back positive. Cases had been trending down since then but have begun to rise in recent weeks, CDC data shows.

Electronic health records data from Epic Research show that Alabama, Nebraska Oklahoma Texas and Wyoming are some of the states currently experiencing the highest number of norovirus cases. Emergency department visits have been on the rise since July, Epic Research data shows. 

Norovirus is a highly contagious virus that is the most common cause of viral gastroenteritis, which is an inflammation of the inside lining of the gastrointestinal tract.

Although it’s often referred to as the “stomach bug” or “stomach flu,” norovirus illness is not related to influenza.

Norovirus is the leading cause of foodborne illness in the U.S., causing 58% of foodborne illnesses each year, and responsible for about 2,5000 outbreaks annually, according to the CDC.

The most common symptoms are nausea, vomiting, stomach pain and diarrhea. Patients, however, can also experience fever, headaches and body aches.

Typically, an infected person will develop symptoms between 12 to 48 hours after being infected. However, norovirus typically resolves quickly and, in most healthy adults, lasts one to three days, according to the CDC.

Health experts say the best way to prevent getting norovirus is to wash hands with warm soap and water for 20 seconds. Hand sanitizer alone does not work well against norovirus.

The CDC says people should wash their hands after using the toilet or changing diapers as well as when eating, preparing or handling food.