Four more residents of a nursing home for the elderly have died of an unknown respiratory illness, bringing the number fatally infected by the disease to 10, Toronto's chief medical officer said Tuesday.

Dr. David McKeown, Toronto's medical officer, said the outbreak at the Seven Oaks Home for the Aged has now affected 84 residents, employees and two visitors to the nursing home on the outskirts of Toronto. Forty residents have been admitted to the hospital.

McKeown insisted that despite the new deaths, the illness was winding down.

"Although the condition of some ill residents has worsened and unfortunately four more have died, others are improving and we are confident this outbreak is under control," McKeown said.

Public health officials have said it may never be possible to determine the exact type of bug responsible for the rash of illnesses, but they have ruled out influenza (search), avian flu (search), SARS (search) and Legionnaire's disease (search).

Anxiety over the outbreak has been exacerbated by fears of SARS — severe acute respiratory syndrome — which claimed 44 lives in Toronto from two outbreaks in the spring of 2003. More than 8,000 people worldwide contracted the illness and 774 people died.

Authorities have said the outbreak, while more serious than average, was not particularly unusual, especially as the flu season approaches.