B737-800
Flight Crew Air Transport Pilot (ATP)
Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter Weather / Turbulence
General Maintenance Action; General Physical Injury / Incapacitation
Human Factors; Weather
At FL360 approximately 80 miles east of COS we began to experience light mountain wave activity. We observed crossing traffic at FL370 at our 12 o'clock position starting to descend on our TCAS. At approximately five miles we got a 'Monitor Vertical Speed' aural warning immediately followed by a RA to 'descend.' As the Captain responded to the RA and initiated a descent; we observed conflicting traffic pass off to our left a couple miles showing +600 feet above us. He was later heard saying he needed lower because he could not hold altitude due to wave activity. Around that time we hit the mountain wave and experienced severe turbulence lasting approx 2-3 minutes. The turbulence caused a few serious roll movements and yaw. Despite still recovering from the RA and trying to hold altitude; we climbed approximately 700 feet in a nose low attitude while in the wave. ATC was told of the deviation during the event. We leveled back at 360 and advised ATC of our RA. Company Dispatch was notified and the aircraft was written up for a severe turbulence encounter.
The aircraft very suddenly encountered severe turbulence. I made a PA and instructed the flight attendants to take their seats. I called the flight attendants and was informed that one in the aft had been thrown to the floor and was hurt. He was in his seat and was told to stay there until the event was over and I could get one of the other flight attendants to him. The Flight Attendant was assessed by the others and we agreed to keep him seated for the duration of the flight and then have EMS meet the airplane on our arrival. On arrival we deplaned and then I escorted the EMS personnel to the rear of the airplane to see the [injured] Flight Attendant. We had EMS evaluate him and then transported [him] to a local hospital for tests and x-rays and a better evaluation.
A B737-800 flight encountered severe turbulence and mountain wave at FL360 and suffered a loss of separation with an aircraft above unable to maintain altitude.  One flight attendant was injured in the two to three minute exposure to the turbulence.