"The water system at Guernsey's King Edward VII Hospital is due to be flushed after "unusually high levels" of the legionella bacteria were found.
A Health and Social Services Department spokesman said nobody had been infected by the bacteria.
To combat its spread the showers have been shut off, water temperature increased and staff have been briefed.
The water is due to be shut off for two hours on Tuesday so the pipes can be flushed before testing on Wednesday.
The King Edward VII has 100 beds and provides rehabilitation, continuing care and day-hospital services for patients with physical needs, mostly those aged more than 55.
The bacteria was discovered during routine testing.
Earlier this year the water systems at the island's Princess Elizabeth Hospital were also found to include high levels of legionella.
It was only reduced to "acceptable levels" after the water system had been treated twice."