Pyelonephritis is a kidney infection usually caused by bacteria that have traveled to the kidney from an infection in the bladder.
Women have more bladder infections (also called urinary tract infections) than men because the distance to the bladder from skin, where bacteria normally live, is quite short and direct. However, the infection usually remains in the bladder.
A woman is more likely to develop pyelonephritis when she is pregnant. Pyelonephritis and other forms of urinary tract infection increase the risk of premature delivery.
A man is more likely to develop the problem if his prostate is enlarged, a common condition after age 50. Both men and women are more likely to develop pyelonephritis if they have any of the following conditions:
An untreated urinary tract infection
Nerve problems that affect the bladder
A bladder tumor
Abnormal backflow of urine from the bladder to the kidneys, called vesicoureteral reflux
An obstruction related to an abnormal development of the urinary tract
Tests or procedures that involve the insertion of an instrument into the bladder also increase the risk of urinary tract infections and pyelonephritis.
Children sometimes develop pyelonephritis because of an abnormality in the bladder that allows urine there to flow backward (reflux) into the ureter, the connection between the kidney and bladder. This can lead to scarring of the kidney.
Rarely, pyelonephritis is so severe that it is life threatening, especially in older people or in people with an impaired immune system.