Norovirus cases are likely rising in Los Angeles, wastewater data shared by the Los Angeles County Department of Public Health shows.

Wastewater sampling -- which collects water from sewers or treatment systems to analyze for pathogens or pollutants -- shows that during the 21-day period ending on Dec. 11, norovirus detection in wastewater rose 154% in Los Angeles and more than 250% in Los Angeles County, according to ABC News' Los Angeles station KABC

"Based on past trends, Public Health expects norovirus activity to rise between November to April, when outbreaks are more common during the cooler months," the health department said in a statement, according to KABC.

US cases of norovirus on the rise with double the rate of positive tests since August

The trend in Los Angeles mirrors those seen nationally. In mid-November, nearly 13% of tests came back positive for norovirus compared to roughly 7% about three months earlier, according to data from the Centers for Disease Control and Prevention (CDC).

More recently, national cases of norovirus appear to be declining, with 9.37% of tests coming back positive as of the week ending Dec. 6, CDC 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 is responsible for about 25,000 outbreaks annually, according to the CDC.

The most common symptoms are nausea, vomiting, stomach pain and non-bloody 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.

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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.

The Los Angeles health department recommends washing fruits and vegetables carefully and thoroughly cooking shellfish because freezing does not kill the virus, according to KABC.

If someone is infected with norovirus, the health department says the person should not prepare food for others for three days after recovering from the illness.