Why do kidney infections occur?

Via urine or blood: Kidney infections are most commonly due to bacteria entering the bladder and work their way up to the kidney. In children commonly due to faulty bladder valves permitting urine to back up to kidney. Many bacteria have hair-like tenticles allowing them to crawl thru bladder valve + up to kidney where they multiply. Less often bacteria can reach kidney via blood stream from skin infection or abscess.

Many possibilities: There are numerous causes of bladder infections (not always involving the kidney), and these can lead to infection in the kidney(s). Inadequate hygiene, improper wiping (for females), sexual intercourse, inadequate fluid intake (referred to commonly as "dehydration"), watery diarrhea, and foreign objects (iuds, vibrators, other probes or inserted objects) and decreased urine flow may contribute.