The city's public health department has issued a medical advisory alerting physicians to report suspected cases of Legionnaires' Disease.

The city confirmed four people contracted the disease. Two of the people, who are aged 55 to 80, were released from hospital last week, while two others remain in hospital for treatment.

Health officials say they are investigating whether there is a link among the individuals. They have not established a link to the downtown area.

Hamilton experienced a Legionnaires' outbreak in the summer of 2006 when five cases were reported. Usually each year Hamilton has up to three reported Legionnaires' cases. Over a three-month period 11 people were reported to have contracted the disease. A 72-year-old Dundas woman died from the disease.

Under fire

Public health officials were under fire from councillors in 2006 for not releasing to politicians and the public important information about the disease.

Legionnaires' Disease is a severe pneumonia that is commonly found in any aquatic environment. The bacteria can survive for several months in a wet environment and multiply in the presence of algae and organic material.

Symptoms of Legionnaires include headaches, muscle pain, high fever, shaking, chills, nausea, vomiting and diarrhea. Doctors say between five and 15 per cent of known cases have been fatal.

The 2006 investigation led to suspected cooling towers in the downtown area. The city did begin testing 25 cooling towers, and found two -- the Art Gallery of Hamilton and a private building -- tested positive for Legionella. Another cooling tower at Sir John A. MacDonald school also tested positive.

It's estimated that about 50 per cent of air conditioning units on private and public buildings are known to carry the bacterium, officials say.