To understand why, you have to know about anatomy. Your urinary tract is made up of your kidneys, bladder, ureters (strawlike tubes), and urethra (the opening from which you urinate). In most urinary tract infections, bacteria enter the urethra, travel to the bladder, multiply, and travel to other parts of the urinary tract (including the kidney).

In women, bacteria gain easy entry to the urethra as it is massaged during intercourse and can cause a bladder infection, Waiting too long before urinating following sexual intercourse will increase the chance of infection, because bacteria that enter the urethra have an opportunity to move farther up the urinary tract. Women who use a diaphragm for birth control are twice as likely to get a urinary tract infection. Pregnancy and postmenopausal changes make you more prone to UTIs, as do congenital abnormalities (urinary tract defects you were born with), any obstructions in the flow of urine (like a kidney stone or enlarged prostate), or having a history of urinary tract infections.

Surprisingly, UTIs may show no symptoms. But usually, if you've got one, you know it. Symptoms strike suddenly, without warning, and include:

A strong desire to urinate
Urinating more often than usual
A sharp pain or burning sensation in the urethra while urinating
Blood in the urine
Feeling that the bladder is still full after you've urinated
Soreness in the abdomen, back or sides (if the infection involves the kidneys)
Chills, fever, nausea, and vomiting (in more serious cases, where the infection involves the kidneys)

If you wait too long to get treatment, the consequences can be serious. Consult a physician if you experience any of the symptoms that are mentioned above. By testing a sample of your urine under a microscope and sending it out to be cultured, your doctor can diagnose your problem.

UTIs are treated with antibiotics. Take all the medication you're prescribed, as directed, even if the symptoms disappear.

Since a UTI is no picnic, you'll be glad to know you may be able to prevent a repeat performance. Here's how:

If you're a woman, you should wipe from front to back after using the toilet to keep bacteria
away from the urethral opening.
Drink plenty of fluids to flush bacteria out of your system.
Empty your bladder as soon as you feel the urge, to give bacteria as little time to multiply as possible.
Empty your bladder as soon as possible after intercourse, even if you don't feel the urge.
Wear cotton underwear, to allow air to circulate freely and discourage the kind of warm, moist
environment in which bacteria thrive.
Avoid using bubble bath if you're prone to UTIs.