In healthy people, urine in the bladder is sterile—no bacteria or other infectious organisms are present. The tube that carries urine from the bladder out of the body (urethra) contains no bacteria or too few to cause an infection. However, any part of the urinary tract can become infected. An infection anywhere along the urinary tract is called a urinary tract infection (UTI).

UTIs are usually classified as upper or lower according to where they occur along the urinary tract. Lower UTIs are infections of the urethra (urethritis) or bladder (cystitis). Some doctors consider prostate infections (prostatitis) to be lower UTIs (see Prostate Disorders: Prostatitis). Upper UTIs are infections of the kidneys (pyelonephritis). In paired organs (such as the kidneys), infection can occur in one or both organs. UTIs can occur in children (see Bacterial Infections in Infants and Children: Urinary Tract Infection in Children(UTI)) as well as in adults.

Causes

The organisms that cause infection usually enter the urinary tract by one of two routes. The most common route by far is through the lower end of the urinary tract—the opening of a man's urethra at the tip of the penis or the opening of a woman's urethra at the vulva. The infection ascends the urethra to the bladder, and sometimes to the kidneys, or both. The other possible route is through the bloodstream, usually to the kidneys.

UTIs are almost always caused by bacteria, although some viruses, fungi, and parasites can infect the urinary tract as well. More than 85% of UTIs are caused by bacteria from the intestine or vagina. Ordinarily, however, bacteria that enter the urinary tract are washed out by the flushing action of the bladder as it empties.

Bacteria: Bacterial infections of the lower urinary tract—the bladder and urethra—are very common, especially among young, sexually active women. Escherichia coli is the most common bacteria to cause a lower UTI. When the person has a kidney stone, Proteus bacteria may be able to grow. Among people between the ages of 20 and 50, bacterial UTIs are about 50 times more common among women than men. In men, the urethra is longer, so it is more difficult for bacteria to ascend far enough to cause an infection. In people older than 50, UTIs become more common among both men and women, with less difference between the sexes.

Factors Contributing to Bacterial Urinary Tract Infections
Ascending Infections Blockage (for example, by stones) anywhere in the urinary tract Abnormal bladder function that prevents proper emptying, such as occurs in neurologic diseases Leaking of the valve-like mechanism between the ureter and the bladder, allowing urine and bacteria to flow backward from the bladder up the ureters, possibly reaching the kidneys (more common among children who have a UTI) Insertion of a urinary catheter or any instrument by a doctor Sexual intercourse Use of a diaphragm with spermicide Presence of an abnormal connection (fistula) between the vagina and the bladder or the intestine and the bladder Among men, prostatitis Blood-borne Infections Infection in the bloodstream (septicemia) Infection of the heart valves (infective endocarditis)

Viruses: The herpes simplex virus type 2 (HSV-2) may infect the urethra, making urination painful and emptying of the bladder difficult.

Fungi: Certain fungi, or yeasts, can infect the urinary tract. This type of infection is often called a yeast infection (yeasts can also cause vaginitis). The fungus Candida is the organism most likely to cause urinary tract yeast infections. Candida frequently infects people who have an impaired immune system or a bladder catheter in place. Rarely, other types of fungi, including those that cause blastomycosis (Blastomyces) or coccidiomycosis (Coccidioides), infect the urinary tract. Fungi and bacteria may infect the kidneys at the same time.

Parasites: A number of parasites, including certain types of worms, can infect the urinary tract.

Trichomoniasis, caused by a type of microscopic parasite, is a sexually transmitted disease that can cause a copious greenish yellow, frothy discharge from the vagina in women. Occasionally, the bladder or urethra becomes infected. Trichomoniasis can infect the urethra in men. It usually causes no symptoms, although it can cause inflammation of the prostate (prostatitis).

Schistosomiasis, an infection caused by a type of worm called a fluke, can affect the kidneys, ureters, and bladder. This infection is a common cause of severe kidney failure among people who live in Africa, South America, and Asia. Persistent bladder schistosomiasis often causes blood in the urine or blockage of the ureters and may eventually result in bladder cancer.

Filariasis, a threadworm infection, obstructs lymphatic vessels, causing lymph fluid to enter the urine (chyluria). Filariasis can cause enormous swelling of tissues (elephantiasis), which, in men, may involve the scrotum.

Last full review/revision September 2007 by Stewart Shankel, MD