JOLIET, Ill. (CBS) — The mystery behind bars at Stateville Correctional Center is solved.

Health officials believe they know what led to a widespread sickness inside the prison, which holds more than 3,000 inmates.

CBS 2’s Chris Martinez reports.

More than 100 prisoners are now sick and bed-ridden after an illness cover over Stateville prison like a wave. Flu-like symptoms crept in just after Christmas Day, overtaking dozens at the same time.

The culprit appears to be Norovirus, says Illinois Department of Corrections spokeswoman Melaney Arnold.

“The Department of Corrections and the Department of Public Health have already started working on outbreak and control efforts,” she says.

Norovirus is highly contagious. The illness may be best known for plaguing cruise ships, where outbreaks have sickened hundreds at a time and passed easily from person to person.

At Stateville, the number of ill stands at about 140. The sick prisoners have been separated from the general population.

Samples from inside are on their way to a state health lab. Their results will confirm if Norovirus is in fact the source.

“There could always be the food transmission, but it could be person-to-person transmission,” Arnold says.

Norvirus usually runs its course within a day or two.