The kidney may become infected as a result of bacterial infection entering the urinary tract through the urethra (the tube that carries urine out of the body) and traveling up through the bladder and ureters (the tubes between the bladder and kidneys) to the kidney.
This causes inflammation of the kidney (pyelonephritis). Symptoms typically appear suddenly and may include intense pain in the lower back (usually only on one side), fever, nausea, vomiting, and frequent, painful urination; the urine may be cloudy, bloodstained, or foul-smelling.
Untreated, pyelonephritis may cause complications such as septicemia or kidney damage. Treatment is with acetaminophen to relieve pain and antibiotics to destroy the bacteria causing the infection. With prompt treatment, most people recover within about 2 weeks.