A urinary tract infection (UTI) is a bacterial infection that affects any part of the urinary tract. Symptoms include frequent feeling and/or need to urinate, pain during urination, and cloudy urine. The main causal agent is Escherichia coli. Although urine contains a variety of fluids, salts, and waste products, it does not usually have bacteria in it, but when bacteria get into the bladder or kidney and multiply in the urine, they may cause a UTI. The most common type of UTI is acute cystitis often referred to as a bladder infection.
An infection of the upper urinary tract or kidney is known as pyelonephritis, and is potentially more serious. Although they cause discomfort, urinary tract infections can usually be easily treated with a short course of antibiotics with no significant difference between the classes of antibiotics commonly used. The most common symptoms of a bladder infection are burning with urination (dysuria), frequency of urination, an urge to urinate, no vaginal discharge, and no significant pain. An upper urinary tract infection or pyelonephritis may also present with flank pain and a fever.
Healthy women have an average of 5 days of symptoms. The symptoms of urinary tract infections may vary with age and the part of the urinary system that was affected. In young children, urinary tract infection symptoms may include diarrhea, loss of appetite, nausea and vomiting, fever, and excessive crying that cannot be resolved by typical measures. Older children on the other hand may experience abdominal pain, or incontinence. Lower urinary tract infections in adults may manifest with symptoms including hematuria (blood in the urine), inability to urinate despite the urge, and malaise.
In young sexually active women, sex is the cause of 75–90% of bladder infections, with the risk of infection related to the frequency of sex. The term “honeymoon cystitis” has been applied to this phenomenon of frequent UTIs during early marriage. In post-menopausal women, sexual activity does not affect the risk of developing a UTI. Spermicide use, independent of sexual frequency, increases the risk of UTIs. Women are more prone to UTIs than men because, in females, the urethra is much closer to the anus and is shorter than in males; furthermore, women lack the bacteriostatic properties of prostatic secretions.
Among the elderly, UTI frequency is roughly equal in women and men. This is due, in part, to an enlarged prostate in older men. As the gland grows, it obstructs the urethra, leading to increased frequency of urinary retention A predisposition for bladder infections may run in families. Other risk factors include diabetes.  While ascending infections are, in general, the rule for lower urinary tract infections, the same is not necessarily true for upper urinary tract infections like pyelonephritis, which may originate from a blood-borne infection.
The following are measures that studies suggest may reduce the incidence of urinary tract infections. * A prolonged course (six months to a year) of low-dose antibiotics (usually nitrofurantoin or TMP/SMX) is effective in reducing the frequency of UTIs in those with recurrent UTIs.  * Cranberry (juice or capsules) may decrease the incidence of UTI in those with frequent infections. Long-term tolerance, however, is an issue. Subsequent research has questioned these findings. * For post-menopausal women intravaginal application of topical estrogens was found to greatly reduce or prevent recurrent cystitis.
As opposed to topical creams, the use of vaginal estrogen plessaries was not as useful as low dose antibiotics. E. coli continues to evolve multi-drug resistance, which bears on the evaluation of appropriate empiric therapy. * Studies have shown that breastfeeding can reduce the risk of UTIs in infants. A number of measures have not been confirmed to affect UTI frequency including: the use of birth control pills or condoms, voiding after sex, the type of underwear used, personal hygiene methods used after voiding or defecating, and whether one takes a bath instead of a shower.
In straight-forward cases, a diagnosis may be made and treatment given based on symptoms alone without further laboratory confirmation. In complicated or questionable cases, it may be useful to confirm via urinalysis, looking for the presence of nitrites, leukocytes, or leukocyte esterase, or via urine microscopy, looking for the presence of red blood cells, white blood cells, and bacteria (with presence of bacteria termed bacteriuria).
Urine culture showing a quantitative count of greater than or equal to 10 colony-forming units (CFU) per mL of a typical urinary tract organism along with antibiotic sensitive is useful to guide antibiotic choice. However, women with negative cultures may still improve with antibiotic treatment. Most cases of lower urinary tract infections in females are benign and do not need exhaustive laboratory work-ups. However, UTI in young infants may receive some imaging study, typically a retrograde urethrogram, to ascertain the presence/absence of congenital urinary tract