Poorly maintained systems increase the risk of bacterial growth. The HSE's ACOP L8, HSG 274 and HTM 04-01 all give guidance on the effective inspection & maintenance measures required to reduce the potential risk of scalding and Legionella.
All fixtures, fittings and valves have a purpose, for example, thermostatic mixing valves (TMV) mix hot & cold water to reduce the hot water temperature to a safe temperature to wash and bathe in. However, these "safe" water temperatures fall within the temperature range that could promote bacterial growth. A failing mixing valve can also increase the risk of scalding.
TMV valves need to be regularly inspected (called a fail safe check) to ensure they are in working order and maintained as required to reduce the potential risk of bacterial growth.