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Overview of Urinary Tract Infections (UTIs)


Talha H. Imam

, MD, University of Riverside School of Medicine

Reviewed/Revised Jan 2024
Topic Resources

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).

Organs of the Urinary Tract

The urinary tract consists of the kidneys, ureters (tubes that carry urine from the kidneys to the bladder), bladder, and urethra (the tube through which urine flows out of the body). These organs may be injured by blunt force (as occurs in a motor vehicle crash or a fall) or by penetrating force (as results from a gunshot or stabbing). Injuries may also occur unintentionally during surgery.

Organs of the Urinary Tract

UTIs are usually classified as upper or lower according to where they occur along the urinary tract, although it is sometimes difficult or impossible for doctors to make such a determination:

Causes of UTIs

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.

Urinary tract infections (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.


Bacterial infections of the lower urinary tract—usually the bladder—are very common, especially among young, sexually active women. Young women also often get bacterial kidney infections, but less commonly than bladder infections. Escherichia coli is the most common bacteria to cause a UTI. 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 men between the ages of about 20 to 50, most UTIs are urethritis Stones in the Urinary Tract Stones (calculi) are hard masses that form in the urinary tract and may cause pain, bleeding, or an infection or block of the flow of urine. Tiny stones may cause no symptoms, but larger stones... read more Stones in the Urinary Tract or prostatitis Prostatitis Prostatitis is pain and swelling, inflammation, or both of the prostate gland. The cause is sometimes a bacterial infection. Pain can occur in the area between the scrotum and anus or in the... read more . 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

Infections beginning in the urinary tract

Infections spread to the urinary tract from the blood (uncommon)

UTI = urinary tract infection.


The herpes simplex virus Herpes Simplex Virus (HSV) Infections Herpes simplex virus infection causes recurring episodes of small, painful, fluid-filled blisters on the skin, mouth, lips (cold sores), eyes, or genitals. This very contagious viral infection... read more Herpes Simplex Virus (HSV) Infections may infect the urethra, making urination painful and emptying of the bladder difficult. Other viral UTIs, such as bladder and kidney infections, do not usually develop unless a person's immune system is impaired (for example, by cancer, HIV/AIDS, or use of a medication that suppresses the immune system).



Trichomoniasis, Trichomoniasis Trichomoniasis is a sexually transmitted infection of the vagina or urethra that is caused by the protozoa Trichomonas vaginalis and that causes vaginal irritation and discharge and sometimes... read more caused by a type of microscopic parasite, is a sexually transmitted infection 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 in men.

Schistosomiasis, Schistosomiasis Schistosomiasis is infection caused by certain flatworms (flukes), called schistosomes. People acquire schistosomiasis by swimming or bathing in fresh water that is contaminated with the flukes... read more 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.

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