AN A320 ENCOUNTERED SEVERE TURB WITH A +40 KTS SPD INCREASE; 500 FT ALT GAIN AND HVY RAIN AT FL360. ONE FLT ATT AND TWO PAX WERE INJURED.

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WE WERE LEVEL AT FL360; 100 NM NE OF DENVER ENRTE FROM ZZZ-PHX. WE HAD THE RADAR ON; SEATBELT SIGN OFF (SINCE FL180 OUT OF ZZZ) AND WE WERE AT CRUISE FOR 1 HR 20 MINS (APPROX). THERE WERE VERY ISOLATED TSTMS ALONG OUR RTE. WE DID NOT HAVE TO DEVIATE. WHILE NOT SEEING ANYTHING ON RADAR; WE ENTERED WHAT LOOKED LIKE CIRRUS CLOUDS. NOT LONG AFTER ENTERING THE CLOUDS; WE ENCOUNTERED SEVERE TURB. WE GAINED 40 KTS; 500 FT; AND FLEW THROUGH HVY RAIN. WE FLEW THROUGH IT IN APPROX 1 MIN. I WAS THE PF. I TURNED OFF THE AUTOPLT AND AUTOTHROTTLES AND CORRECTED THE ALT BACK TO FL360. THE FO TURNED ON THE SEATBELT SIGN AND NOTIFIED ATC. IN THE CABIN; 2 PAX RPTED (WHAT APPEARED TO BE) MINOR INJURIES ALONG WITH THE 'B' FLT ATTENDANT. THE FLT ATTENDANT AND THE AFT CART FLEW NEARLY TO THE CEILING. DRINKS; FOOD AND OTHER ITEMS NOT SECURED WERE THROWN THROUGHOUT THE CABIN. AFTER WE REGAINED CTL; WE FLEW THE REMAINDER OF THE FLT WITHOUT FURTHER INCIDENT. PARAMEDICS MET US AT THE GATE UPON ARR AND ATTENDED THE INJURED. WE HIT UNEXPECTED SEVERE TURB. I WAS THE PF; AND I TURNED OFF THE AUTOPLT AND AUTOTHROTTLES AND CORRECTED THE ALT BACK TO FL360. THE FO TURNED ON THE SEATBELT SIGN AND NOTIFIED ATC. SENT NOTIFICATION TO HAVE PARAMEDICS MEET US UPON ARR. IT SEEMS THAT A TSTM BUILT SO FAST THAT WE DID NOT PICK IT UP ON RADAR.