CABIN ATTENDANT IS INJURED WHEN E135 ENCOUNTERS WAKE TURBULENCE WHILE AT CRUISE AT 11000 FT MSL.

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WE WERE JUST GETTING READY TO LEVEL OFF AT 11000 FEET; IT WAS A SMOOTH RIDE; AND I HAD JUST TURNED OFF THE SEAT BELT SIGN. OUR FA HAD ALREADY BEGUN SERVICE AT THAT POINT. A FEW SECONDS LATER; WE WERE HIT BY WAKE TURBULENCE. THERE WAS NO AIRCRAFT VISIBLE NEARBY; THERE WAS NONE DISPLAYED ON TCAS; NOR WERE WE ALERTED TO ANY WAKE BY ATC. WE CAN ONLY ASSUME THAT THE WAKE HAD LINGERED THERE FOR QUITE SOME TIME AFTER AN AIRCRAFT HAD PASSED LONG BEFORE. THE AIRCRAFT ROCKED RAPIDLY ONCE TO THE LEFT; THEN THE RIGHT; THEN IT WAS SMOOTH FROM THAT POINT ON FOR THE REST OF THE FLIGHT. IMMEDIATELY AFTER THE ENCOUNTER; WE CALLED OUR FA ON THE INTERPHONE; AND HE SAID THAT HE HAD HIT HIS HEAD; BUT FELT FINE. WE CONTINUED THE FLIGHT NORMALLY TO LANDING IN SAN; AND THE FA WAS ABLE TO CONTINUE ALL OF HIS NORMAL DUTIES. ONCE WE GOT ON THE GROUND; OUR FA INFORMED US THAT HIS HEAD HAD STEADILY BEGUN HURTING WORSE SINCE THE INITIAL EVENT; AND THAT HE FELT HE SHOULD GET SOME MEDICAL ATTENTION IN ORDER TO ENSURE THERE WERE NO SERIOUS INJURIES. WE AGREED; AND NOTIFIED ALL CONCERNED PARTIES THAT HE WOULD BE REMOVED FROM THE RETURN FLIGHT. THIS WAS JUST BAD LUCK; WRONG PLACE/WRONG TIME. WE ALWAYS STRIVE TO AVOID KNOWN WAKE TURBULENCE; BUT SOMETIMES WAKE CAN STRIKE WITHOUT WARNING.