Three cases of legionnaires' disease have been found in Marlborough over the past month - two more than the total number of cases in the region last year.
The illness normally occurs after inhalation of mist containing the Legionella bacteria.
Nelson Marlborough Medical Officer of Health Ed Kiddle says none of the three patients have been in contact with potting mix - where the bacteria is frequently found - prompting an examination of other possible sources, such as water cooling towers.
Mr Kiddle says businesses with cooling towers are required to have a programme in place to monitor for and treat Legionella, however the Public Health Service has further treated a number of towers in the area as a precaution.
He says three cases in one month is serious as typically only one case of legionnaires' disease is reported in the region per year.
Those most at risk of infection are people over 50 years of age, smokers, and anyone with immune suppression.
Symptoms of the disease include fever, fatigue, muscle pains and pneumonia.
