In simple terms, the medical term pyelonephritis means kidney infection. "Pyelo-" is from the Greek "pyelos", meaning basin or pelvis1, and "nephritis" means an inflammation of the kidney2. Pyelonephritis is an inflammation of the kidney and the pelvis of the kidney; caused by bacterial infection. For an anatomical diagram and detailed information on kidney anatomy, please see "Anatomy of the Kidney and Ureter" from Washington State University.
Urinary tract infections are classified as "upper" or "lower". An infection of the kidneys is an upper urinary tract infection, an infection of the bladder and urethra is a lower urinary tract infection. Pyelonephritis may be caused by an ascending infection from the lower urinary tract or from the blood (hematogenous spread). An ascending infection is the most common route of infection.
For more information about Urinary Tract Infections, please see:
What are the signs seen with a urinary tract infection?
pyelo-. Dictionary.com. Dictionary.com Unabridged (v 1.1). Random House, Inc. http://dictionary.reference.com/browse/pyelo- (accessed: July 31, 2007).
nephritis. Dictionary.com. Online Etymology Dictionary. Douglas Harper, Historian. http://dictionary.reference.com/browse/nephritis (accessed: July 31, 2007).