An unusual jump in late-winter flu cases has prompted hospitals in the Chicago area to restrict visitors to protect very ill patients from picking up the flu.

The Illinois Department of Public Health says officials statewide are seeing a flu increase.

The Chicago-area flu season usually lasts from October to May and peaks in early January. Health professionals say this year’s late-season increase might be due in part to milder temperatures.

Advocate Christ Medical Center in Oak Lawn banned visitors younger than 18 and any adult visitor with flu-like symptoms.

Hospitals throughout the area have enacted similar restrictions