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A urinary tract infection (UTI) is an infection that begins in your urinary system. UTIs limited to your bladder can be painful and annoying. But serious consequences can occur if the infection spreads to your kidneys.

Women are most at risk of developing a UTI. In fact, half of all women will develop a UTI during their lifetimes, and many will experience more than one.

The urinary system is composed of the kidneys, ureters, bladder and urethra. All play a role in removing waste from your body. The kidneys, a pair of bean-shaped organs in your upper posterior abdomen, filter waste from your blood. Tubes called ureters carry urine from your kidneys to your bladder, where it is stored until it exits the body through the urethra. All of these components can become infected, but most infections involve the lower tract — the urethra and the bladder.

Antibiotics are the typical treatment for urinary tract infections. But you can take steps to reduce your chance a getting a UTI in the first place.

Not everyone with a UTI develops recognizable signs and symptoms, but most people have some. These can include:

• A strong, persistent urge to urinate
• A burning sensation when urinating
• Passing frequent, small amounts of urine
• Blood in the urine (hematuria) or cloudy, strong-smelling urine

Each type of UTI may result in more specific signs and symptoms, depending on which part of your urinary tract is infected:

• Acute pyelonephritis. Infection of your kidneys may occur after spreading from an infection in your bladder. Kidney infection can cause upper back and flank pain, high fever, shaking chills, and nausea or vomiting.
• Cystitis. Inflammation or infection of your bladder may result in pelvic pressure, lower abdomen discomfort, frequent, painful urination and strong-smelling urine.
• Urethritis. Inflammation or infection of the urethra leads to burning with urination. In men, urethritis may cause penile discharge.

Urinary tract infections typically occur when bacteria enter the urinary tract through the urethra and begin to multiply in the bladder. The urinary system has infection-fighting properties that inhibit the growth of bacteria and is designed to keep out such microscopic invaders. However, certain factors increase the chances that bacteria will enter the urinary tract, take hold and multiply into a full-blown infection.

Having bacteria in the urine doesn't always signify an infection. Some people, especially older adults, may have bacteria in the urine without any signs or symptoms of infection. This condition, known as asymptomatic bacteriuria, doesn't need treatment.

Cystitis may occur in women after sexual intercourse. But even girls and women who aren't sexually active are susceptible to lower urinary tract infections because the anus is so close to the female urethra. Most cases of cystitis are caused by Escherichia coli (E. coli), a species of bacteria commonly found in the gastrointestinal tract.

In urethritis, the same organisms that infect the kidney and bladder can infect the urethra. In addition, because of the female urethra's proximity to the vagina, sexually transmitted diseases (STDs), such as herpes simplex virus and chlamydia, also are possible causes of urethritis.

In men, urethritis often is the result of bacteria acquired through sexual contact. The majority of such infections are caused by gonorrhea and chlamydia.

Some people appear to be more likely than others to develop UTIs. Up to half of all women will develop a bladder infection over a lifetime. A key reason is their anatomy. Women have a shorter urethra than men have, which cuts down on the distance bacteria must travel to reach the bladder.

Women who are sexually active tend to have more UTIs. Sexual intercourse can irritate the urethra, allowing germs to more easily travel through the urethra into the bladder. Women who use diaphragms for birth control also may be at higher risk, as are women who use spermicidal agents. After menopause, UTIs may become more common because tissues of the vagina, urethra and the base of the bladder become thinner and more fragile due to loss of estrogen.

Other risk factors include:

• Anything that impedes the flow of urine, such as an enlarged prostate in men or a kidney stone
• Diabetes and other chronic illnesses that may impair the immune system
• Medications that lower immunity, such as chronic cortisone therapy or chemotherapy for cancer
• Prolonged use of tubes (catheters) in the bladder

A woman's immune system may play a role in her risk of recurrent UTIs. Bacteria may be able to attach to cells in the urinary tract more easily in women lacking protective factors that normally allow the bladder to shed bacteria. More research is needed to determine the exact factors involved and how such factors can be manipulated to benefit women with frequent UTIs.

If you have symptoms of a urinary infection, contact your doctor promptly. If your doctor suspects you have a UTI, he or she may ask you to turn in a urine sample to determine if pus, red blood cells or bacteria are present in your urine. Laboratory analysis of the urine (urinalysis), sometimes followed by a urine culture, can reveal whether you have an infection. Although no simple test can differentiate between an upper and lower urinary tract infection, the presence of fever and flank pain indicate that the infection likely involves your kidneys.

When treated promptly and properly, UTIs rarely lead to complications. But left untreated, a urinary tract infection can become something more serious than a set of uncomfortable symptoms.

Untreated UTIs can lead to acute or chronic kidney infections (pyelonephritis), which could permanently damage your kidneys. Young children and older adults are at the greatest risk of kidney damage due to UTIs because their symptoms are often overlooked or mistaken for other conditions. Women who have UTIs while pregnant may also have an increased risk of delivering low birth weight or premature infants.

If your symptoms are typical of a UTI and you're generally in good health, antibiotics are the first line of treatment. Which drugs are prescribed and for how long depends on your health condition and the type of bacteria found in your urine. Drugs most commonly recommended for simple UTIs include amoxicillin (Amoxil, Trimox), nitrofurantoin (Furadantin, Macrodantin), trimethoprim (Proloprim) and the antibiotic combination of trimethoprim and sulfamethoxazole (Bactrim, Septra). Make sure your doctor is aware of any other medications you're taking or any allergies you have. This will help him or her select the best treatment.

Usually, UTI symptoms clear up within a few days of treatment. But you may need to continue antibiotics for a week or more. Take the entire course of antibiotics recommended by your doctor to ensure that the infection is completely eradicated. For an uncomplicated UTI that occurs when you're otherwise healthy, your doctor may recommend a shorter course of treatment, such as taking an antibiotic for three days. But whether this short course of treatment is adequate to treat your UTI depends on your particular symptoms and medical history.

If you have recurrent UTIs, your doctor may recommend a longer course of antibiotic treatment or a self-treatment program with short courses of antibiotics at the outset of your urinary symptoms. For infections related to sexual activity, your doctor may recommend taking a single dose of antibiotic after sexual intercourse.

For severe UTIs, hospitalization and treatment with intravenous antibiotics may be necessary. When recurrences are frequent or a kidney infection becomes chronic, your doctor will likely refer you to a doctor who specializes in urinary disorders (urologist) or a doctor whose specialty is kidneys (nephrologist) for an evaluation to determine if urologic abnormalities may be causing the infections.

You can take steps to reduce your risk of urinary tract infections. Women in particular may benefit from the following:

• Drink plenty of liquids, especially water. Cranberry juice may have infection-fighting properties. However, don't drink cranberry juice if you're taking the blood-thinning medication warfarin. Possible interactions between cranberry juice and warfarin can lead to bleeding.
• Urinate promptly when the urge arises. Avoid retaining your urine for a long time after you feel the urge to void.
• Wipe from front to back. Doing so after urinating and after a bowel movement helps prevent bacteria in the anal region from spreading to the vagina and urethra.
• Empty your bladder as soon as possible after intercourse. Also, drink a full glass of water to help flush bacteria.
• Avoid potentially irritating feminine products. Using deodorant sprays or other feminine products, such as douches and powders, in the genital area can irritate the urethra.

UTIs can be painful, but you can take steps to ease your discomfort until antibiotics clear the infection. Follow these tips:

• Drink plenty of fluids. Drinking plenty of water dilutes your urine and may help flush out bacteria. Avoid coffee, alcohol, and soft drinks containing citrus juices and caffeine until your infection has cleared. They can irritate your bladder and tend to aggravate your frequent or urgent need to urinate.
• Use a heating pad. Sometimes a heating pad placed over the abdomen can help minimize feelings of bladder pressure or pain.

If you have recurrent bladder infections, let your doctor know. Together you can determine a strategy to reduce recurrences and the discomfort UTIs can bring.