Kidney infection. Artwork of a section through the urinary system showing an infection of a kidney (pyelonephritis, upper left). The affected kidney is sectioned to show the formation of pus (yellow). Pyelonephritis may be either acute or chronic. The acute form is usually caused when bacteria causing cystitis spread up the ureters (yellow, down left and right) from the bladder (red, lower centre). It causes fever and back pain. Treatment is with antibiotic drugs. The chronic form usually starts in childhood, and may lead to permanent kidney damage. The descending aorta artery (red) and ascending vena cava vein (blue) are seen between the kidneys.