Norovirus cases are rising in the U.S., CDC data shows

• It's also the number one cause of foodborne illness in the U.S., according to the health agency.

Driving the news: There were 91 norovirus outbreaks reported during the week of Dec. 5, according to data recently published by the CDC.

• The agency defines an outbreak as an two or more similar illnesses from a common exposure that is either "suspected or laboratory-confirmed to be caused by norovirus."
• The latest figures represent a significant bump from the 69 outbreaks reported the week of Nov. 28 and the 32 outbreaks documented on the week of Nov. 21.
• It's also much higher than in years past; CDC data indicates that the highest number of outbreaks reported in the first week of December was 65, per AP.

The big picture: Norovirus outbreaks can occur across all age groups, though children under the age of five and the elderly adults are especially vulnerable to severe infections, according to the CDC.

• About 19 million to 21 million people in the U.S. contract norovirus each year, the Scientific American noted.
• The virus often spreads in "confined spaces" like cruise ships, day care centers, and college dorms, the journal added.

Go deeper: Your wastewater could reveal norovirus outbreaks