Acute pyelonephritis is one of the most common renal diseases. In this disorder, sudden inflammation is caused by bacterial invasion. It occurs mainly in the interstitial tissue and the renal pelvis and occasionally in the renal tubules. It may affect one or both kidneys. With treatment and continued follow-up care, the prognosis is good and extensive permanent damage is rare.

Pyelonephritis occurs more commonly in females than in males, probably because the shorter urethra and the proximity of the urinary meatus to the vagina and rectum allow bacteria to reach the bladder more easily. Females also lack the antibacterial prostatic secretions that males produce.

Typically, the infection spreads from the bladder to the ureters and then to the kidneys, commonly through vesicoureteral reflux. Vesicoureteral reflux may result from congenital weakness at the junction of the ureter and the bladder. Bacteria refluxed to intrarenal tissues may create colonies of infection within 24 to 48 hours.