The legionnaires’ disease outbreak in London, Ont., continues to grow as the Middlesex-London Health Unit reported a 25th case connected to the rare virus on Monday.

The MLHU gave its first update on the outbreak since last Wednesday as it noted that four people remain in hospital who had caught the disease. The health unit was first informed about the cluster of cases on July 24.

Last week, the organization said that a number of cooling towers in an area of southeast London where most of those who had been infected either lived or worked had been tested, with evidence of legionella being found in three.

Legionella is the bacteria that leads to the development of legionnaires’ disease and is often found in warm water sources such as hot water tanks and air conditioners.

Dr. Joanne Kearon, acting medical officer of health, has said the investigation will still take some time, and it’s possible the source won’t be found “for a variety of reasons.”

“A five-kilometre radius is still quite a large geographic area, and so that does mean that the investigation may take some time,” she said last week. “We have already been urging people operators to be looking at their maintenance records and … systems and ensuring that it is sanitized and maintained up to date. And so, by the time that we get to sampling them, the location may already have been sanitized and the risk removed.”

On Monday, the MLHU said it is still looking for the source of the outbreak, which has cost the life of one person over the age of 50.

It noted that any locations that have tested positive for legionella bacteria have been sanitized, removing any threat to area residents.

Health Canada says the disease is uncommon as, for every 100 people who come into contact with the bacteria, fewer than five will catch Legionnaire’s disease.

In Canada, fewer than 100 people contract the virus every year, although Health Canada says that number could be much higher with many cases going unreported.

The disease was first discovered in 1976 after 29 people who had contracted the virus died at an American Legion Convention in Philadelphia.

The symptoms of legionnaires’ disease include a dry cough and shortness of breath, which can lead to pneumonia, and begin two to 10 days after encountering the bacteria.

— with files from Global News’ Benjamin Harrietha and Amy Simon