The source of a growing legionnaires’ disease outbreak that killed an elderly woman has been narrowed down to two Melbourne suburbs.

The woman aged in her 90s arrived in hospital severely unwell and died on Tuesday night.

Victorian Chief Health Officer Clare Looker said there were 60 confirmed cases and a further 10 suspected cases linked to the outbreak, with 66 people admitted to hospital as of 3pm on Thursday.

The source was likely to be a cooling tower in the Laverton North and Derrimut area in Melbourne’s northwest, Looker told reporters on Friday.

Health department officials have disinfected 41 of the 100 cooling towers in the area, with test results due back in coming days.

Looker said cases may continue to rise with a “short, sharp peak” expected, but was hopeful the source had already been stopped.

“There’s a high probability that we have already visited the tower and disinfected it,” she said.

“That is really driven by some pretty compelling epidemiology and crossover of where many of our cases are visited.”

The deceased woman lived in the broader area of concern and authorities believe most cases were exposed to the disease between July 5 and 20, she said.

“I wouldn’t like people to panic. I think that this is something that we can control and we hopefully have already done so,” Looker said.

“But I would urge them to remain vigilant to symptoms for now.”

Legionnaires’ disease is caused by the legionella bacteria, found in natural bodies of water as well as spas, warm water systems, potting mix and artificial systems that use water for cooling.

Looker said the outbreak was unusual due to the high load of the bacteria at the source and because cold weather in mid-July may have caused bacteria to spread several kilometres from the source.

“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.

Looker said authorities were keeping an open mind about whether the outbreak was linked to a recent large chemical factory fire in Derrimut, but no similar situations had been documented.

Symptoms of legionnaires’ disease can include a chest infection, aches, headache, fever, cough and chills.

People with underlying diseases, including those who are immunocompromised, elderly people and smokers, are at greater risk of infection.

Royal Melbourne Hospital physician Megan Rees said, while some patients can become very unwell, others may have few symptoms.

“There is no transmission between people. We do sometimes see more than one case in a household, but it’s usually because they’ve been exposed to the same source,” Rees said.

“So there is no risk for patients of spreading it to their loved ones.”

She urged anyone with symptoms to see a general practitioner as the disease can be easily detected through a urine test.