Urinary Tract Infection
Infection of the normally sterile urinary tract, usually by ascending gut bacteria, whose site of infection (bladder versus kidney) and host factors dictate how it presents and how aggressively it must be treated.
First principles
The urinary tract is defended by flow, and infection is a breach of that defence
Above the urethra the urinary tract is sterile. Its principal defence is not immunological but mechanical: the continuous downward flow of urine flushes out organisms before they can establish, aided by urine's low pH and osmolality and by mucosal barriers. Most infections are ascending: gut flora, chiefly Escherichia coli, colonise the perineum and climb the urethra. Anything that impairs flushing (obstruction, incomplete emptying, stones, catheters) or shortens the ascent lets bacteria establish, which is the single idea from which the risk factors and the treatment principles all follow.
Educational content pending clinical review. Not medical advice.