Acute pyelonephritis (acute pyelonephritis) is an acute purulent inflammation caused by bacteria invading the renal pelvis, calyces, and renal parenchyma. The course of disease does not exceed 6 months. There are two ways of infection: ① Ascending infection, bacteria enter the renal pelvis from the ureter, and then invade the renal parenchyma. 70% of acute pyelonephritis originates from this route. ② Hematogenous infection. Bacteria enter the renal tubules from the bloodstream and invade the renal pelvis from the renal tubules, accounting for about 30%. Most are staphylococcal infections. Urinary tract obstruction and urinary stagnation are the most common causes of acute pyelonephritis. Simple pyelonephritis is rare.