While cruising in smooth air conditions with no clouds in the area and with the autopilot on and the seat belt sign off the airplane had a brief encounter with what the flight crew described as wake turbulence. The momentary turbulence resulted in a sharp roll deviation of about 15 to 20 degrees. The turbulence lasted for less than five seconds then returned to smooth air conditions. One of the two flight attendants who had been standing in the rear galley was seriously injured and the other flight attendant sustained a minor injury. No significant turbulence had been forecast, and the flight crew had not overheard any airplanes in the area reporting turbulence. Immediately after the encounter, the airplane continued in smooth air conditions.