Pyelonephritis most often occurs as a result of urinary tract infection, particularly in the presence of occasional or persistent backflow of urine from the bladder into the ureters or kidney pelvis (vesicoureteric reflux). Pyelonephritis can be further classified as follows: - Acute uncomplicated pyelonephritis (sudden development of kidney inflammation)
- Chronic pyelonephritis (a long-standing infection that does not clear)
Although cystitis (bladder infection) is common, pyelonephritis occurs much less often. The risk is increased if there is a history of cystitis, renal papillary necrosis, kidney stones, vesicoureteric reflux, or obstructive uropathy. The risk is also increased when there is a history of chronic or recurrent urinary tract infection and when the infection is caused by a particularly aggressive type of bacteria. Acute pyelonephritis can be severe in the elderly and in people who are immunosuppressed (for example, those with cancer or AIDS). | 
Internal Medicine, Renal Disease
