Pathogenesis of urinary tract infections (UTIs) is not well-understood. In this paper, we review the current understanding of UTIs, particularly in relationship to individuals using intermittent catheterization. Relationships exist between the human host, infectious agent and the environment. In the human host, the urethra connects the bladder to potential infectious agents on the perineum. A high-pressure zone exists within the urethra at a point where the urethra passes through the urogenital diaphragm. This zone creates a natural barrier to ascent of organisms colonized in the distal urethra and the bladder itself has natural defences against invading organisms. The interaction of host defences with bacteria (infectious agent) determines whether or not the bacteria persist. A small number of bacteria and some types of bacteria are controlled more effectively by natural bladder defence mechanisms and frequent bladder emptying than a large number of bacteria. Escherichia coli, coliforms and enterococci are considered common bacterial causes of UTIs and are found in high numbers on the perineum. Intermittent catheterization is an effective way of bladder emptying but as an invasive procedure it remains a risk factor in the development of UTI.