A B757 EXPERIENCED LOSS OF CABIN PRESSURE AT CRUISE ALTITUDE. THE FLT CREW DECLARED AN EMERGENCY; EXECUTED AN EMERGENCY DESCENT; AND DIVERTED TO THE NEAREST SUITABLE AIRPORT.

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I WAS THE CAPT ON A COMMERCIAL ACR FLT. MY FO AND I DEPARTED ZZZ1 WITH 1 MEL ITEM OUTSTANDING FOR AN INOP R RECIRCULATION FAN. DURING CLBOUT AND LEVELOFF WE WERE IN AND OUT OF CLOUDS AND DEVIATING SLIGHTLY FOR TSTMS. ENG ANTI-ICE WAS ON FROM APPROX FL240 TO LEVELOFF. SHORTLY AFTER REACHING OUR ASSIGNED CRUISE FLT LEVEL OF FL360 THE ACFT EXPERIENCED A NOTICEABLE DEPRESSURIZATION BUMP ACCOMPANIED BY IMMEDIATE 'CABIN ALT' AURAL AND EICAS WARNINGS. THERE WERE NO OTHER WARNINGS; CAUTIONS; OR STATUS MESSAGES ASSOCIATED WITH THIS EVENT; BUT CABIN ALT WAS CLBING AT APPROX 1000-1500 FPM; PASSING THROUGH APPROX 10000 FT; AND WAS VERY UNCOMFORTABLE. AT THAT POINT WE IMMEDIATELY INITIATED THE QRH PROC FOR A 'CABIN ALT WARNING' AND DECLARED AN EMER. MAX CABIN ALT ATTAINED DURING THE EVENT WAS APPROX 13000 FT. WE ALSO REQUESTED AND WERE GRANTED AN EMER DSCNT TO 10000 FT BY CTR. DURING THE DSCNT AND LEVELOFF WE ALSO REQUESTED AND WERE GRANTED COURSE DEVS AS NECESSARY TO AVOID TSTMS IN THE AREA. DURING THE ENTIRE EVENT THERE WERE NO INDICATIONS OF ANY LOSS OF SEPARATION; NO ATC TFC CALLS; AND A TCAS SCAN INDICATED NO OTHER ACFT IN THE VICINITY. THE REMAINDER OF THE DSCNT AND LEVELOFF WERE UNEVENTFUL. DURING THE DSCNT WE COMMUNICATED WITH CABIN CREW WHO DID A MAGNIFICENT JOB KEEPING THE PAX INFORMED AND PROVIDING FOR THEIR WELFARE THROUGHOUT THE ENTIRE EVENT. AFTER WE LEVELED OFF WE WERE ABLE TO REGAIN CTL OF THE CABIN AND ALL COCKPIT INDICATIONS RETURNED TO NORMAL. AT THAT POINT WE CANCELED THE EMER AND CONTACTED THE COMPANY. AFTER CONSULTATION WITH BOTH DISPATCH AND MAINT THE DECISION WAS MADE TO DIVERT TO ZZZ2. THERE WERE NO FURTHER ABNORMAL INDICATIONS DURING THE APCH AND LNDG PHASE OF FLT. SUBSEQUENT DISCUSSIONS WITH MAINT INDICATED THE ACFT HAD A PRIOR HISTORY OF PRESSURIZATION MALFUNCTIONS WHICH HAD RESULTED IN A RECENT L AIR CYCLE MACHINE CHANGE. THOSE DISCUSSIONS ALSO INDICATED THAT THE MOST PROBABLE CAUSE OF THE DEPRESSURIZATION WAS A FAULTY 'AFT CARGO DOOR SEAL' FAILURE THAT ONLY OCCURRED AT HIGH DIFFERENTIAL PRESSURE. THAT WAS ALSO THE REASON FOR THE LACK OF OTHER EICAS WARNINGS AS WELL AS THE REESTABLISHMENT OF CABIN CTL AT LOWER ALTS/DIFFERENTIAL PRESSURE. THE ACFT WAS SUBSEQUENTLY FERRIED BACK TO A MAINT STATION FOR FURTHER MAINT ACTION.