Mayo Clinic – Kidney infection typically occurs when bacteria enter your urinary tract through the tube that carries urine from your body (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 happen — for instance, if you have an artificial joint or heart valve that becomes infected.
May 28, 2016 – Most urinary tract infections(UTIs) involve only the bladder and urethra (the lower urinary system). Pyelonephritis results when a UTI progresses to involve the upper urinary system (the kidneys and ureters).
Most cases of pyelonephritis are complications of common bladder infections. Bacteria enter the body from the skin around the urethra. They then travel up the urethra to the bladder.
Sometimes, bacteria escape the bladder and urethra, traveling up the ureters to one or both kidneys.
Pyelonephritis is a potentially serious kidney infection that can spread to the blood, causing severe illness. Fortunately, pyelonephritis is almost always curable with antibiotics.
The urethra is much shorter in women than in men, which is one reason why women are more vulnerable to UTIs and pyelonephritis.