In many cases, bacteria first travels to the urethra. When bacteria multiply, an infection can occur. An infection limited to the urethra is called urethritis. If bacteria move to the bladder and multiply, a bladder infection, called cystitis, results. If the infection is not treated promptly, bacteria may then travel farther up the ureters to multiply and infect the kidneys. A kidney infection is called pyelonephritis. A kidney infection (pyelonephritis), is typically more serious than a bladder infection (cystitis). Pyelonephritis is often associated with fever, back pain and nausea which may require hospitalization. Pyelonephritis can result in damage and scarring of the kidneys over time.

Microorganisms called Chlamydia and Mycoplasma may also cause UTIs in both men and women, but these infections tend to remain limited to the urethra and reproductive system. Unlike bacteria such as E. coli, Chlamydia and Mycoplasma may be sexually transmitted, and infections require treatment of both partners.

The urinary system is structured in a way that helps ward off infection. By far, women are more frequently affected by UTIs. In men, the prostate gland produces secretions that slow bacterial growth. In both sexes, immune defenses also prevent infection. But despite these safeguards, infections can still occur. Pyelonephritis is uncommon because the ureters and bladder normally prevent urine from backing up toward the kidneys and the flow of urine from the bladder helps wash bacteria out of the body. Maintaining adequate hydration may help patients prone to UTIs.

Some people are more prone to getting a UTI than others. Any abnormality of the urinary tract that obstructs the flow of urine (a kidney stone, for example) sets the stage for an infection. In men, an enlarged prostate gland also can slow the flow of urine and impede prober bladder emptying, thus raising the risk of infection.

A common source of infection is indwelling catheters, or tubes, placed in the urethra and bladder. A person who cannot void or who is unconscious or critically ill often needs a catheter that stays in place for a long time. Some people, especially the elderly or those with nervous system disorders who lose bladder control, may need a catheter for life. Bacteria on the catheter can infect the bladder, so hospital staff take special care to keep the catheter clean and remove it as soon as possible.

People with diabetes have a higher risk of a UTI because of changes in the immune system. Any other disorder that suppresses the immune system raises the risk of a urinary infection.

UTIs may occur in infants, both boys and girls, who are born with abnormalities of the urinary tract, which sometimes need to be corrected with surgery. UTIs are rarer in boys and young men. In adult women, though, the rate of UTIs gradually increases with age. Scientists are not sure why women have more urinary infections than men. One factor may be that a woman’s urethra is short, allowing bacteria quick access to the bladder. Also, a woman’s urethral opening is near sources of bacteria from the anus and vagina. For many women, sexual intercourse seems to trigger an infection, although the reasons for this linkage are unclear.

According to several studies, women who use a diaphragm are more likely to develop a UTI than women who use other forms of birth control. Recently, researchers found that women whose partners use a condom with spermicidal foam also tend to have growth of E. coli bacteria in the vagina.