MADISON, Wis. -- Local health officials said on Wednesday that several of the dozens who fell ill during a science fiction convention at Madison's Concourse Hotel this weekend were developing symptoms soon after arriving.

Officials with the Public Health Department of Madison & Dane County said that those showing symptoms were already incubating the illness.

The illness, which still hasn't been identified, then spread to other attendees through shared food, common surfaces and personal contact, officials said. About 50 people in total became sick at the convention.

Officials said that they were collecting samples to determine the cause of the stomach-flu-like ailment.

Steve Zanoni, the hotel's general manager, said on Tuesday that they have determined the sickness didn't come from food provided by the Concourse. Zanoni said that the hotel has been scrubbed from top to bottom since the illnesses were reported.