The U.S. health officials announced on Monday that as many as 200 people who attended a catered event at a downtown Seattle's Russell Investments Center have become ill with norovirus.

The health officials have closed all food vendors in a downtown Seattle office building, including a Starbuck location, as they continue to investigate the norovirus outbreak.

The catered event held on December 1 in the cafeteria at the Russell Investments Center in downtown Seattle was attended by approximately 600 people.

The Seattle & King County Public Health reported 150 cases showing norovirus symptoms, and another 50 cases were reported directly to the local health agency. Two sicken people have been hospitalized and eight people required emergency room attention.

The outbreak is expected to grow. in Seattle Times, Dr. Meagan Kay, a medical epidemiologist in the health department's communicable disease section said that the health officials anticipate that the number is likely low.

Dr. Kay said that norovirus is very common, and more than a dozen outbreaks are confirmed or probable in the county each year. Norovirus can spread person to person very quickly. Dr. Kay added that norovirus can spread easily through anything such as door knobs and handrails.

According to Centers for Disease Control and Prevention (CDC), norovirus can spread by an infected person, contaminated water, or by touching contaminated surfaces. Norovirus causes stomach pain, nausea, vomiting, and diarrhea. Norovirus illness can be serious, especially for children and older people. The symptoms can lead to a serious dehydration.

The event at the Russell Investments Center was catered by Bon Appetit Management Co., the firm that supplies restaurant services in 33 states. Its operation in the Seattle office building was shuttered, but services at other locations remain open.

But health officials prohibit the food service employees working at the Russell Investments Center to work at other venues.

According to Reuters, Bon Appetit's spokesperson said that the company have worked with its food safety experts to disinfect the surfaces in the company's facilities and have taken all other necessary steps to ensure food safety.

The company said that it is as eager as anyone to learn precisely how and when the outbreak began.

The Russell Investments Center was cleaned thoroughly over the weekend to combat the germs, including cleaning door knobs and other surfaces, and disinfecting in part to address vomiting in restrooms. The Seattle office building houses high profile tenants including Zillow and the Seattle food safety law firm Marler Clark.