Normally, urine is sterile. It is usually free of bacteria, viruses, and fungi but does contain fluids, salts, and other metabolic waste products. An infection occurs when tiny organisms, usually bacteria from the digestive tract, cling to the opening of the urethra and begin to multiply. The urethra is the tube that carries urine from the bladder to outside the body. Most infections arise from one type of bacteria, Escherichia coli (E. coli), which normally lives in the colon.
In many cases, bacteria first travel to the urethra. When bacteria multiply, an infection can occur. An infection limited to the urethra is called infectious urethritis. If bacteria move to the bladder and multiply, a bladder infection, called infectious cystitis, results. If the infection is not treated promptly, bacteria may then travel further up the ureters to multiply and infect the kidneys. A kidney infection is called infectious pyelonephritis.