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, although it is sometimes difficult or impossible for doctors to make such a determination:
Some doctors also consider infections of the urethra (urethritis) and prostate (prostatitis) to be lower UTIs. In paired organs (such as the kidneys), infection can occur in one or both organs. UTIs can occur in children as well as in adults.
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 or prostatitis. In people older than 50, UTIs become more common among both men and women, with less difference between the sexes.
The herpes simplex virus 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 drug that suppresses the immune system).
Certain fungi, or yeasts, can infect the urinary tract. This type of infection is often called a yeast infection (yeasts can also cause inflammation of the vagina [vaginitis]). The fungus Candida is the organism most likely to cause urinary tract yeast infections (candidiasis). Candida frequently infects people who have an impaired immune system or a bladder catheter in place. Fungi and bacteria may infect the kidneys at the same time.
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 in men.
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.