A bacterial infection involving the kidneys. It is more common in females, although it affects both genders and all ages. Kidney infections in males are more likely if the bladder does not drain easily (one cause of this problem is a large prostate.) Bladder infections are common in women, and the typical cause of a kidney infection in a female is migration of bacteria from the bladder infection. The infection travels up the tube (ureter) that drains into the bladder from the kidneys.