The pool and spa at Springs Club were closed in August after two people were diagnosed with the disease.
A further 76 confirmed or probable cases of the less serious Pontiac Fever, also caused by the legionella bug, were identified.
Questionnaires will be sent to 800 visitors in an effort to work out why some became ill and others did not.
The facilities remain closed while investigations into the cause continue.
'Risk factors'
All of those affected have recovered or are recovering, according to the Health Protection Agency (HPA).
Dr Kirsty Foster of the HPA, said: "Outbreaks of Pontiac Fever are not common, and it is not well understood why some people get Pontiac Fever and others develop Legionnaires' disease, which is why we want to understand more about the infection.
"This week we are writing to over 800 people who used the health club between 12 and 23 of August to ask them to fill out a questionnaire.
"This includes people who we know were ill and those who weren't as this will help us to identify the risk factors which influenced why some people developed the infection and some didn't.
"Once we have gathered and analysed the data we hope to publish the findings of the study so that other doctors, scientists and local authorities across the country can learn more about this infection."
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