Health officials are treating the incidence of Legionnaires' disease in the Glasgow and Lanarkshire health board areas as an outbreak.
Of the seven people to have been confirmed as having the disease, three have died.
Public health protection units are investigating to identify the source of the disease that cannot be spread from person to person.
Legionnaires' disease is a form of pneumonia caused by inhaling small droplets of water suspended in the air, which contain the legionella pneumophilia bacteria.
They are usually spread through aerosols produced from water, such as air conditioning systems and showers.
Poorly-maintained water and plumbing systems are often blamed for increasing the risk of the bacteria developing. It can grow in water temperatures between 20 and 45ºc, although most healthy people who are exposed to the bacteria do not become ill.
Instead, it mainly affects people who are susceptible because of age, illness, smoking and other pre-existing health conditions.