The Health and Safety Executive (HSE), which launched an investigation into the incident at Walton Lifestyles in Walton-on-the-Naze in November 2016, identified a failure in the council's system for managing the risk of legionella.
Colchester Magistrates' Court was told that the regular user of the centre's facilities, fell seriously ill and was taken to hospital where he remained for 18 days. He was diagnosed with Legionnaires' Disease, sepsis, pneumonia and chronic kidney failure.
The investigating officer said the council's failures could easily have led to a fatality and Walton Lifestyles users were put at significant risk.
Water samples were taken from the men's shower and tested positive for legionella bacteria.
The HSE found that the council had failed to adequately manage the water systems at a number of leisure centres in the district, including Walton Lifestyles, Dovercourt Lifestyles and Clacton Leisure Centre.
Significant failings included not having suitable and sufficient legionella risk assessments for the leisure facilities. Staff were not adequately trained, and a lack of monitoring meant the failures went unnoticed for several months.
HSE inspector Tania van Rixtel said: "Controls such as maintaining water temperatures, regular flushing of low-use outlets and adequate cleaning are all necessary in order to reduce the risk of legionella developing."
Van Rixtel said that, given the number of people who use the facilities, the potential legionella risk to the public would have been significant.
The council, which pleaded guilty to breaching s 3(1) of the Health and Safety at Work Act, was fined £27,000 and must pay £7,500 costs.
Chief executive Ian Davidson apologised to the individual who was affected and reassured customers that the council's legionella management systems had been overhauled.
"I would like to reassure the public, and particularly our leisure centre customers, that our leisure centres are safe," he said.
He added that the council took its health and safety responsibilities seriously and said: "We deeply regret that standards fell below the high level that we hold ourselves to, and we know which the public expect from us-¦ We fully accept the fine issued by the court."
