Almost 50 people are suspected to have come down with a "flu-like" illness caused by the legionella bug after using a health club in Sunderland.
The pool and spa area of the Springs Club, off Wessington Way, Castletown, have been closed while health experts investigate the cause of the outbreak.
The Health Protection Agency said the bug, pontiac fever, which has made 46 people ill, was not life-threatening.
A spokesman for the club said the pool and spa had been closed "voluntarily."
A spokeswoman for the Health Protection Agency, which found the bug in the club's spa, said the illness was a far milder form of Legionnaires' disease and symptoms lasted between two and five days.
Muscle aches
Experts from the Sunderland Teaching Primary Care Trust and Sunderland City Council are also helping to investigate the outbreak.
Dr Kirsty Foster of the protection agency said: "We are waiting for test results before we confirm the diagnosis but the symptoms we are seeing are not consistent with Legionnaires' disease, which causes pneumonia.
"But they do suggest a similar illness called pontiac fever which is caused by the same Legionella organism.
"It an unpleasant but milder illness and causes flu-like symptoms which last a few days.
"It's not known why the Legionella organism sometimes causes Legionnaires' Disease and sometimes pontiac fever.
"We are continuing to investigate the source of the infection and the health club, which is co-operating fully with the investigation, has voluntarily closed their spa pool, swimming pool and steam room until further notice."
Patients with the fever are advised to take paracetamol or ibuprofen to ease fever and muscle aches.