The prevalence of bacteriuria depends on age and gender. Among neonates, positive cultures are found in about 1% of both boys and girls. During school age, the prevalence among boys is as low as 0.03%, in comparison with 1% to 2% among girls. The prevalence among women increases by 1% of the population per decade; throughout the childbearing years, the prevalence is 2% to 4%, and by age 50 years, it has reached 5% to 10%. Elderly men are almost as likely to have bacteriuria as women because of the high incidence of prostatic and other urologic diseases and subsequent instrumentation in this group. Prevalence in these older age groups reaches 15%.

The greater susceptibility of younger women and girls can be explained anatomically in that a short urethra allows easier access to the bladder, so that colonization by perineal organisms is facilitated. Risk increases with local trauma associated with sexual activity and the relaxation of the pelvic supporting structures with age. Anatomic changes may also explain the higher prevalence of bacteriuria (4% to 7%) among pregnant women. Alternatively, because users of birth control pills also have an increased risk, the higher prevalence may also reflect estrogen-mediated dilation of the urethra. Use of spermatocides also increases risk in sexually active women. Changes in bacterial flora are thought to contribute to the increased prevalence of asymptomatic bacteriuria in perimenopausal women.

The prevalence of bacteriuria is even higher in diabetic women. The relative risk in comparison with nondiabetic women is approximately three. The risk among diabetic men is not increased. It must be kept in mind that prevalence figures indicate the extent of bacteriuria at a single point in time. Because risk factors are shared by many, and bacteriuria frequently resolves spontaneously as well as after therapy, the cumulative prevalence of bacteriuria is higher. By age 30 years, approximately 25% of women have experienced symptoms consistent with urinary tract infection (UTI).

Structural abnormalities (including obstruction of the urethra or ureters), significant vesicourethral reflux, neurogenic bladder dysfunction, and the presence of foreign bodies such as stents are important additional risk factors for bacteriuria. A maternal history of UTI and early age at onset in premenopausal women suggest a genetic component to risk, as does family history conferring increased risk of recurrent UTI. Urinary tract instrumentation increases risk of UTI in hospitalized patients; a chronic indwelling catheter typically leads to persistent bacteriuria.