The number of norovirus cases in California is rising, as 32 confirmed outbreaks have already occurred since October, while only nine outbreaks were reported at this time last year, the California Department of Public Health (CDPH) announced Wednesday.

Norovirus infection, also known as stomach flu, is the most common cause of gastroenteritis in the U.S., resulting in about 19 to 21 million gastroenteritis cases annually and 570 to 800 people dying from the disease every year. Outbreaks frequently happen from November to April, according to the CDPH.

Its symptoms include abdominal pain, diarrhea, low-grade fever, body aches, nausea and headache. The virus can also cause vomiting, which is why norovirus infection is sometimes referred to as winter vomiting disease. The symptoms begin to show 12 to 48 hours after a person is exposed to the virus, and they last from one to three days.

The virus is highly contagious and can spread quickly in crowded and closed environments like cruise ships, schools, hospitals, daycare centers and nursing homes. Many of the recent outbreaks occurred in nursing homes and similar facilities, while other outbreaks affected schools.

"It's very readily transmissible," Dr. William Schaffner from Vanderbilt University School of Medicine told The Los Angeles Times. "That also contributes to the nursing home outbreaks, because those people are all confined and so they get repeated exposure to people who are sick or who are going to be sick or have recovered from sickness and are still capable of transmitting the virus."

The disease spreads from person to person by direct contact, eating contaminated food or touching a contaminated surface. Infected persons remain contagious even up to two weeks from the time of recovery.

"One of the most important things you can do to avoid norovirus and other illnesses this holiday season is to wash your hands frequently with soap and running water for at least 20 seconds," Dr. Karen Smith, CDPH Director and State Public Health Officer, said in the news release. "This is especially important after using the bathroom, changing diapers, and before preparing or eating food. Hand sanitizers are not effective against norovirus."