The major cause of this kidney disorder is bacteria entering the urethra and making its way up into the bladder and then travels to the kidneys. This means that the cause is almost always the same (although it can in rare cases happen from infections elsewhere in the body spreading to the kidneys).
Everyone is potentially susceptible to this problem, but some people are at greater risk. Here's link showing risk factors -- you can also navigate from there to causes, symptoms, etc.
The most frequent causes of pyelonephritis are the gram-negative aerobic bacteria, among which the most common is Escherichia coli.
All organisms that cause acute cystitis may also cause acute pyelonephritis. Proteus species are particularly important because they produce an enzyme called urease, which causes alkaline urine and favors the formation of renal lithiasis (kidney stones).
Pyelonephritis usually affects one kidney, both kidneys are affected only occasionally.