About a third of the 600 people who went to a catered event last week in a Seattle office building indicated they came down with norovirus, according to multiple published reports.

The Seattle Times reported that about 200 people who attended the party last Tuesday at Russell Investments Center in the city’s downtown area fell ill. A public health official told the newspaper that eight people were treated in emergency rooms and two were hospitalized.

According to the Centers for Disease Control and Prevention (CDC), the highly contagious norovirus is the top cause of foodborne-illness outbreaks in the United States, sickening 19-21 million people, hospitalizing 56,000-71,000 and killing 570-800 annually. The virus, which affects the stomach and intestines, causes stomach pain, nausea, diarrhea and vomiting. It’s acquired by eating tainted food or water, touching contaminated surfaces or interpersonally.

Al Jazeera reported crews worked to decontaminate the building over weekend and that food service remains closed by order of the health department. The news agency reported authorities were still trying to determine what exactly caused the outbreak, as a few cases of the illness had been reported before Tuesday’s party. The CDC said that norovirus can spread rapidly in closed environments, and this time of the year is generally when most United States outbreaks occur.

The CDC recommends washing hands, properly preparing food like fruit, vegetables and seafood and disinfecting any contaminated surfaces as three ways to combat the spread of norovirus.