Kidney infection typically occurs when bacteria enter your urinary tract through the urethra and begin to multiply. Bacteria from an infection elsewhere in your body also can spread through your bloodstream to your kidneys. Kidney infection is unusual through this route, but it can occur in some circumstances — for instance, when a foreign body, such as an artificial joint or heart valve, gets infected. Rarely, kidney infection results after kidney surgery.
Bacteria in the urine don't always signify an infection. Some people, especially older adults, may have bacteria in the bladder that don't cause any signs or symptoms or harm, and therefore doesn't require treatment. This condition is known as asymptomatic bacteriuria.