That statement followed the death of one person from Legionnaires' disease and 18 further cases in Stoke-on-Trent since 24 July.
A hot tub displayed in a store in the town is thought to be the "probable" source after samples from it were found to match those taken from the patients.
The water in hot tubs is kept at between 30 and 35 degrees, close to body temperature, which is the ideal environment for legionella bacteria to grow.
So you don't have to be sitting in the hot tub to inhale the bacteria.
Professor Nick Phin, head of Legionnaires' Disease at the Health Protection Agency, says an outbreak of the disease in the Netherlands in 1999 illustrates how far the spray can travel.
"Over 140 people were affected at a Dutch flower show by two spa baths being exhibited. People were just passing through and breathing in the fine spray."
But not everyone who walked past the hot tubs was infected by the legionella bacterium, he says.
"You have to be susceptible to it. There was a very low attack rate of 0.2% in the Holland example."