Pyelonephritis is a bacterial infection of the kidney. It can be serious because of the important function of the kidneys and because the infection may enter the bloodstream. Kidney infections are much more common in women than men.
Most kidney infections result from lower urinary tract infections, usually bladder infections.
The urethra is the tube that empties urine from the bladder. A woman's urethra is short, about 2 inches long, and is just above the vagina. Because of the location and short length of the female urethra, women are more likely to have bladder infections than men. Lower urinary system infections may spread to the kidneys, causing pyelonephritis. When you have a bladder infection, the usual mechanism that prevents urine from flowing back to the kidneys does not work properly.