It’s common for some showers to be rarely used, this allows the bacteria counts to grow. Flushing infrequently used showers every week will help reduce the risk and the HSE stipulate that they should be descaled & disinfected every 3 months. Shower Head Plus is an ideal product to use since descales and disinfects at the same time.
A simple legionella test will demonstrate if your flushing and cleaning regime is being effective.
Don’t forget a pot wash spray in your kitchen is exactly the same as a shower!