Abstract

This study analyzes an ongoing, long-term legionellosis outbreak (April 2024–January 2026) at a general hospital to identify building water system factors contributing to prolonged transmission. Utilizing as-built drawing reviews and on-site inspections, the study evaluated the water system’s operational and contamination status. Results indicated that contamination was localized within the hot water system, driven by poor circulation (caused by pump capacity deficits and air-locks), thermostatic mixing valves, and stagnant low-use zones. In particular, supply temperature control failures from un-retrofitted equipment created critical temperature management gaps during repairs. The findings underscore that prolonged outbreaks stem from interconnected environmental and engineering vulnerabilities, highlighting the urgent need for timely water system retrofits and strategic monitoring in high-risk facilities.