People have been asked to flush taps, drain unused garden hoses and put commercial screen wash in cars to stop the spread of Legionnaires’ disease.
The warning from Public Health Wales comes as a cluster of 11 cases has been identified in the Vale of Glamorgan in the past 12 months.
Legionnaires’ disease is a lung infection caught by inhaling droplets of water that contain the bacteria.
Officials say there is “no evidence” the cases around Barry are linked.
“Although Legionnaires’ disease is rare, it can be a potentially life-threatening illness,” said Dr Gwen Lowe, of Public Health Wales.