An infection of the kidney and the ducts that carry urine away from the kidney (ureters). Pyelonephritis can be further classified according to the extent and severity of damage to the kidneys. Types include:- reflux nephropathy- acute uncomplicated pyelonephritis (sudden development of kidney inflammation)- complicated UTI (chronic pyelonephritis).

Pyelonephritis most commonly occurs as a result of urinary tract infection (cystitis), particularly in the presence of transient(occasional) or persistent backflow of urine from the bladder into the ureters or kidney pelvis (vesicoureteric reflux ).Although cystitis (bladder infection) is common, pyelonephritis occurs much less often. The incidence of pyelonephritis isapproximately 3 to 7 out of 10,000 people. The risk is increased if there is a history of cystitis, renal papillary necrosis, kidneystones, vesicoureteric reflux, or obstructive uropathy . The risk is also increased when there is a history of chronic or recurrenturinary tract infection, and when the infection is caused by a particularly virulent bacteria.Acute pyelonephritis can be severe in the elderly and in people who are immuno suppressed (for example, those with cancer or AIDS) .

Prompt and complete treatment of cystitis (bladder infection) may prevent development of many cases of pyelonephritis.Chronic or recurrent urinary tract infection should be treated thoroughly because of the chance of infection of the kidneys.