On April 5, 2010, about 1930 central daylight time, a Saab 340B, N437XJ, operated by Mesaba Airlines, Inc., as Delta Connection Flight 3271, and piloted by an airline transport pilot, encountered turbulence while in cruise flight near Bradley, Illinois. The sole flight attendant received serious injuries when she fell during the turbulence encounter. There was no damage to the airplane and no injuries to the other crew or passengers on the flight. The 14 Code of Federal Regulations Part 121 flight was operating in instrument meteorological conditions and was on an instrument flight rules flight plan. The airplane had departed from the Detroit Metropolitan Wayne County Airport at 1827 and was en route to the General Downing - Peoria International Airport when the event occurred. The airplane was approaching its destination airport and the flight crew was attempting to make course corrections to avoid entering cumulus clouds on the airplane's flight path. At this point, the seatbelt sign was illuminated and the passengers were seated with their seat belts fastened. As the flight crew deviated around clouds, a buildup of clouds was unavoidable and the airplane entered the clouds. As the airplane entered the clouds, a brief turbulence encounter was experienced. At this time the flight attendant had just completed collecting garbage after the beverage service and was preparing to sit in her seat. The flight attendant stated that she was raised up off of the floor and her head hit the cabin roof. She was thrown back down to the floor and landed on her buttocks. The flight attendant was able to complete her duties with pain, and sought medical attention after the flight. It was later determined that she suffered a broken tail bone. The severity of the turbulence encountered was described as moderate by the operator and severe by the flight attendant.