DRG FLT, UNITED FLT 108 RECD INFO OF A BOMB THREAT. DUE TO A SHORT TIME LIMIT, SPECIFIED IN THE THREAT, THE FLT DIVERTEDTO DETROIT & THE SLIDES WERE USED TO EVACUATE THE ACFT; 8 OF 10 SLIDES DEPLOYED NMLY & WERE USED. THE SLIDE AT DOOR 4R DIDN'T INFLATE & WASN'T USED. THE FLT ATTENDANT (F/A) AT DOOR 5R PERCEIVED THAT THE SLIDE (SLIDE/RAFT) AT THAT LCTN WAS INFLATING SLOWLY. SHE PULLED WHAT SHE THOUGHT WAS THE MANUAL INFLATION HANDLE (MIH) & HEARD A HISSING SOUND. SHE THEN SENT 2 HELPERS (PAX) DWN THE SLIDE. AS THEY EGRESSED, THE SLIDE SEPD FM THE ACFT & FELL TO THE GND; 1 OF THE 2 RECD A SERIOUS BACK INJURY. AS OTR SLIDES WERE USED, 2 OTR PAX RECD SERIOUS LEG INJURIES & 16 PAX RECD MINOR INJURIES. AN INV REVEALED THE F/A AT DOOR 5R INADVERTENTLY PULLED THE RELEASE HANDLE (RH) WHEN SHE ATMTD TO USE THE MIH. BOTH HANDLES WERE MADE OF CANVAS MTRL & WERE ABOUT THE SAME SIZE. THE MIH WAS RED WITH A PIECE OF WHITE MTRL MARKED 'PULL' IN RED LTRS; THE RH WAS WHITE & WAS ALSO MARKED 'PULL' IN RED LTRS. NO REASON WAS FND FOR THE SLIDE AT DOOR 4R NOT TO ENFLATE.