Pyelonephritis is a type of urinary tract infection (UTI) that generally begins in your urethra or bladder and travels to one or both of your kidneys.
A kidney infection requires prompt medical attention. If not treated properly, a kidney infection can permanently damage your kidneys or the bacteria can spread to your bloodstream and cause a life-threatening infection.
Bacteria that enter your urinary tract through the tube that carries urine from your body (urethra) can multiply and travel to your kidneys. This is the most common cause of kidney infections.
Bacteria from an infection elsewhere in your body also can spread through your bloodstream to your kidneys.
Although it's unusual to develop a kidney infection, it can happen — for instance, if you have an artificial joint or heart valve that becomes infected.
Rarely, kidney infection results after kidney surgery.