Urinary tract infections (UTIs) are exceedingly common in both the outpatient and inpatient settings. They occur in patients of all ages, affecting females throughout life and males at each end of the age spectrum. In 2000, there were an estimated 8.27 million outpatient physician visits with UTI as the primary diagnosis. In addition, UTIs are the most common nosocomial infection and the leading cause of gram-negative bacillary sepsis in hospitalized patients. The phrase urinary tract infection encompasses a broad array of diagnoses, including cystitis, pyelonephritis, asymptomatic bacteriuria, complicated infections associated with nephrolithiasis or bladder catheters, and recurrent infections. The appropriate management of a patient with a UTI entails the consideration of several factors, including the patient's age and sex, the presence of underlying diseases or pregnancy, the history and timing of prior UTIs, the differentiation between cystitis and pyelonephritis, and the expected microbial uropathogen involved.

The delineation of upper versus lower tract infection is essential to understanding the approach to therapy. Lower urinary tract infection is infection involving the bladder (cystitis) and describes the syndrome of dysuria, pyuria, increased urinary frequency, or urgency. Upper urinary tract infection, or pyelonephritis, is infection involving the bladder and kidney that classically presents with fever and flank pain, with or without the symptoms of lower tract infection. The pathogenesis of most upper and lower UTIs is related to the ability of microorganisms to establish colonization in the periurethral area and subsequently ascend into the urinary tract, thus causing infection.