More than 390 people in the midwestern U.S. state of Kansas have reported illness in an outbreak of norovirus linked to a theater restaurant, according to a statement released by the Kansas Department of Health and Environment (KDHE).

The KDHE said it initially surveyed individuals attending the New Theatre Restaurant in Overland Park of Kansas from Jan. 17, but it now decided to expand the survey to all persons who attended performances since Jan. 15.

"So far during KDHE's investigation, more than 390 individuals have reported illness and attended the New Theatre Restaurant from Friday, Jan. 15 to present (Thursday)," the statement said.

Four people who became ill had laboratory specimens that confirmed norovirus, but the KDHE said it's still investigating the source of the illness, which is very contagious and spread through person-to-person contact, contaminated food or water, or by touching contaminated surfaces.

Norovirus infection symptoms include diarrhea, vomiting, nausea and stomach pain. A person develops symptoms 12 to 48 hours after being exposed to the virus and most people get better within one to three days. The best way to prevent norovirus is proper handwashing.

Norovirus infection, which is common in winter, sickens about 19 to 21 million persons across the United States each year.