kidney infection is a type of urinary tract infection (UTI). A kidney infection may begin in the tube that carries urine from the body (urethra) or in the bladder. The infection can travel to one or both kidneys. A kidney infection is also called pyelonephritis. A kidney infection needs prompt medical treatment. If not treated properly, an infection can cause lasting damage to the kidneys. Or the bacteria can spread to the bloodstream and cause a dangerous infection.