Pyelonephritis: Bacterial infection of the kidneys. Pyelonephritis can be acute or chronic, and it is most often due to the ascent of bacteria from the bladder up the ureters to infect the kidneys. Symptoms include flank (side) pain, fever, shaking chills, sometimes foul-smelling urine, frequent and urgent need to urinate, and general malaise. Tenderness is elicited by gently tapping over the kidney with a fist (percussion). Diagnosis is made via urinalysis, which reveals white blood cells and bacteria in the urine. Usually there is also an increase in circulating white cells in the blood. Treatment involves use of appropriate antibiotics. Often called simply pyelo.