Public health officials in Hamilton, Ont., are searching for the source of legionella that sickened seven of the city's residents this month.

It may be impossible to find a common source among the respiratory cases, Dr. Elizabeth Richardson, the city's medical officer of health, said Thursday.

Legionnaires' disease is a type of pneumonia that is caused by legionella, a bacterium found mainly in warm water environments. Symptoms include high fever, chills and a cough.

"Although legionella is often associated with a single point of exposure such as an institution or a spa, these cases are community-based and present us with a more complex investigation," Richardson said in release.

Air conditioning systems, hot tubs and water distribution systems are common sources of exposure in outbreaks, according to public health officials.

In five of the seven cases, the people lived in lower Stoney Creek or Lower East Hamilton, and geography is the only common thread so far.

As of Wednesday, two were in hospital and were expected to recover. The patients range in age from their 20s to 70s.

Anyone in Hamilton with symptoms should seek medical attention, the city's health department urged. Medical exams and tests are needed to determine whether the pneumonia is caused by legionella.

Legionnaires' is not a communicable disease.