Pyelonephritis is a bacterial infection of the kidney. It is potentially serious because of the important function of the kidneys and because the infection may also be in the bloodstream. Kidney infections are much more common in women than men.
Most kidney infections result from lower urinary system infections, usually of the bladder.
When a person has a bladder infection, the usual mechanism that prevents urine from flowing back to the kidney does not work properly. This malfunction can allow infected urine to move into the kidney. If your urinary system is abnormal, it is more likely that a lower urinary tract infection will spread to the kidney. The urethra is the tube that empties urine from the bladder. In men the urethra extends the full length of the penis.