Bacterial, especially colonic flora such as Escherichia coli, Proteus and Klebsiella.
Most cases are due to ascending infections from urine flow abnormalities.
Urinary outflow obstruction results in urine stasis providing an environment for bacterial proliferation. Obstruction also predisposes to vesicoureteral reflux which can enable bacteria to reach the kidney. Most cases are related to pre-existing urine outflow abnormalities such as pregnancy, tumor, calculi, prostatic hypertrophy, neurogenic bladder and vesicoureteral reflux. Women are more prone than men, possibly due to differences in urethral anatomy. Renal parenchymal involvement tends to be regional, not universal, commonly upper and lower poles.