Definition of Pyelonephritis:

Pyelonephritis is an infection of the kidney and the ureters, the ducts that carry urine away from the kidney.

Causes, incidence, and risk factors:

Pyelonephritis most often occurs as a result of urinary tract infection, particularly when there is occasional or persistent backflow of urine from the bladder into the ureters or an area called the kidney pelvis. See: Vesicoureteric reflux

Pyelonephritis can be sudden (acute) or long-term (chronic).

• Acute uncomplicated pyelonephritis is the sudden development of kidney inflammation.
• Chronic pyelonephritis is a long-standing infection that does not go away.

Pyelonephritis occurs much less often than a bladder infection, although a history of such an infection increases your risk. You're also at increased risk for a kidney infection if you have any of the following conditions:

• Backflow of urine into the ureters or kidney pelvis
• Kidney stones
• Ostructive uropathy
• Renal papillary necrosis

You are also more likely to get a kidney infection if you have a history of chronic or recurrent urinary tract infection, especially if 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).