A B757 EXPERIENCES SEVERE TURB RESULTING IN A CABIN ATTENDANT INJURY.

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WHILE ENRTE TO ORD; WE EXPERIENCED SEVERE TURB AT FL330. AFTER DSNDING OUT OF FL370; ATC ADVISED US OF OCCASIONAL MODERATE CHOP AT FL330. SHORTLY THEREAFTER; I NOTIFIED THE FLT ATTENDANTS OF THE POSSIBILITY OF SOME TURB AND TO BE READY TO SIT DOWN WHEN IT STARTED TO GET ROUGH; ONCE WE WERE LEVEL AT FL330. THE RIDE WAS FAIRLY SMOOTH. WE THEN RECEIVED A CLRNC TO DSND TO FL260. THE FO STARTED TO DSND AND IT STARTED TO GET A LITTLE BUMPY. A FEW SECONDS AFTER THAT IS WHEN WE ENCOUNTERED THE SEVERE TURB. WE WERE UNABLE TO MAINTAIN OUR ALT OR OUR DSCNT. OUR INSTS INDICATED A VERT SPD OF APPROX 2000 FPM CLB AND THE ACFT CLBED TO APPROX FL339. ATC ASKED US IF WE WERE HAVING TROUBLE MAINTAINING ALT AND OF COURSE WE SAID YES. A SECOND OR TWO LATER; THE ACFT STARTED TO DSND AT A RAPID RATE WITH A LOT OF CHOP AS WELL. WE DSNDED TO APPROX FL310 WHEN THINGS STARTED TO SETTLE DOWN AND WE CONTINUED OUR DSCNT TO FL260. WE CHKED WITH THE FLT ATTENDANTS AFTER THE EVENT AND THEY INFORMED US THAT ALL PAX WERE SEATED AND THAT AT THAT TIME WE HAD NO INJURIES. WE CONTINUED TO ORD AND THE REST OF THE FLT WAS UNEVENTFUL. AFTER WE BLOCKED IN; A FLT ATTENDANT RPTED TO ME THAT SHE HAD INCURRED AN INJURY TO HER R WRIST AND HER BACK. WE DISCUSSED THIS ISSUE WITH DISPATCH AND MAINT. SUPPLEMENTAL INFO FROM ACN 598723: ACFT PITCH ATTITUDE VARIED BY 5 DEGS AND BANK ANGLE WAS WITHIN 30 DEGS. THE RADAR DURING THIS TIME SHOWED NOTHING MORE THAN LIGHT RAIN. THE TOTAL EVENT LASTED BTWN 30-40 SECONDS.