On September 30, 1999, about 1315 Eastern Daylight Time, an Airbus A319-131, N821UA, operated by United Airlines as flight 1509, encountered turbulence over Sparta, New Jersey. The airplane was not damaged. Two flight crew members, 2 flight attendants, and 40 passengers were not injured; however, a third flight attendant sustained a serious neck injury. Visual meteorological conditions prevailed, and an instrument flight rules flight plan had been filed for the scheduled passenger flight that departed Boston International Airport (BOS), Boston, Massachusetts, at 1205. The flight was conducted under 14 CFR Part 121 and was destined for Dulles International Airport (IAD), Dulles, Virginia. The captain reported light chop to occasional light turbulence at flight level 310 en route to IAD. About 15 minutes from IAD, a flight attendant notified the captain that another flight attendant had been thrown against door 2L and became unconscious. After a few minutes, she regained consciousness and was administered oxygen by a third flight attendant. The captain declared a medical emergency and requested that paramedics meet the flight upon landing at IAD. According to the captain, no significant weather was forecast for the flight. According to the injured flight attendant, the seat belt sign was "on", but no announcement had been made for the flight attendants to be seated. Two of the flight attendants had returned to their seats and the injured flight attendant was locking a cart in the galley when she "felt a bump." As she was proceeding to her seat, the airplane encountered another "bump" and she was thrown against the assist handle on the 2L door. She fell to the floor and became unconscious for about 2 minutes. The other two flight attendants provided her with oxygen and placed blankets under her head. The injured flight attendant remained in the same position on the floor until the flight landed. After landing, the injured flight attendant was transported by helicopter to a local hospital. According to a United Airlines representative, the flight attendant suffered from a herniated C-6 disk. She remained in the local hospital for 5 days, and was then transported to a different hospital for surgery to repair the disk. Weather reports indicated no forecasted turbulence along the route of flight. At flight level 310, winds were from the west/southwest at 50-70 knots. No significant vertical or horizontal wind shear was forecast at that altitude, and pilot reports for the area indicated no turbulence at cruise levels.