Kidney Infection. Pyelonephritis is iinfection in a kidney. It can be caused either by bacteria moving from the skin up the urinary tract and invading the kidney or by bacteria floating around in your blood until they take up residence in your kidney. Either has been given as an explanation for kidney infections and I have not seen anything to say one is more likely than the other.
Ascending infection. Of the urinary tract that rises from the bladder, up through the ureters and into the pelvis (pyelus) of the kidney and into the kidney proper.