Showers can become a breeding ground for bacteria, particularly dangerous because the purpose of a shower is to spray water, creating aerosols.
The L8 recommends quarterly cleaning of showerheads, or ‘as necessary’.
Urban myth used to blame Legionella infection in people returning from holidays on ‘mucky foreign hotels’.
An alternative theory is that their first shower in their own bathroom is an equally likely cause. A small colony of bacteria, which may have been in the showerhead or component parts, would have had uninterrupted opportunity to multiply whilst they were away.