An inflammatory process affecting the kidney. The cause is most often bacterial, but may also be fungal in nature. Signs and symptoms may include fever, chills, flank pain, painful and frequent urination, cloudy or bloody urine, and confusion. Inflammation of the KIDNEY involving the renal parenchyma (the NEPHRONS); KIDNEY PELVIS; and KIDNEY CALICES. It is characterized by ABDOMINAL PAIN; FEVER; NAUSEA; VOMITING; and occasionally DIARRHEA.