An outbreak of legionnaires' disease in Victoria has grown to 71 cases including an elderly woman who died in hospital, but residents near the suspected source are urged not to panic.
The confirmed cases were in addition to a further seven suspected patients, with most needing hospital admission, health authorities confirmed late on Friday.
One woman aged in her 90s arrived at hospital severely unwell and died on Tuesday night.
A cooling tower in the Laverton North and Derrimut area, in Melbourne's west, is believed to be the source of the outbreak, Victoria's chief health officer Clare Looker said on Friday.
Health department officials were due to have disinfected 56 of the 100 cooling towers in the two suburbs of concern by the end of the day.
Dr Looker said cases were expected to have a "short, sharp peak" and she was hopeful the source had been stopped.
"There's a high probability that we have already visited the tower and disinfected it," she said.
The deceased woman lived in the affected area and authorities believe most patients were exposed between July 5 and 20.
"I wouldn't like people to panic," Dr Looker said.
"This is something that we can control and we hopefully have already done so but I would urge them to remain vigilant to symptoms."
Dr Looker said the outbreak was unusual due to the high load of bacteria at the source and because cold weather in mid July might have caused it to spread several kilometres.
"We consider the area of highest risk to be the 500 metres around that cooling tower, so it doesn't mean a case has actually visited the factory or the site of interest, but that they've been in that 500m radius around the site," she said.
People with underlying diseases, including those who are immunocompromised, elderly people and smokers are at greater risk of infection.
Hudson Institute of Medical Research chief executive Elizabeth Hartland said it was an unusual outbreak since so many patients have ended up in hospital.
"They seem to be quite seriously ill and that might suggest they've inhaled a larger dose of legionella or there's something inherently more virulent about this particular strain," Prof Hartland said.
The microbiologist said authorities might never pinpoint the source but it was a serious concern as Victoria already had some of the world's strictest cooling tower safety rules.
"Something is amiss and that needs to be investigated," she said.
Victorian Premier Jacinta Allan offered condolences to the elderly woman's family and urged anyone with symptoms to seek immediate medical help.
"This is a really terrible illness and can cause, as we've seen, death and serious illness," she told reporters on Friday.
Royal Melbourne Hospital physician Megan Rees said the disease was not transmitted person-to-person and could be easily detected with a urine test.
"We do sometimes see more than one case in a household but it's usually because they've been exposed to the same source," she said.
"There is no risk for patients of spreading it to their loved ones."
Legionnaires' disease is caused by the legionella bacteria, found in natural bodies of water, spas, warm water systems, potting mix and artificial systems that use water for cooling.
Symptoms can include a chest infection, aches, headache, fever, cough and chills.
