Pyelonephritis is usually caused by a bacterial infection. The most common pathogen involved is Escherichia coli (E. coli), which typically ascends from the lower urinary tract. Other bacteria that can cause pyelonephritis include Proteus, Klebsiella, and Enterococcus. Risk factors include urinary obstruction, vesicoureteral reflux, diabetes mellitus, and immunosuppression.
