The Health Protection Agency said the latest case brings the total to 21. The patient, who is being treated at the University Hospital of North Staffordshire, is a man in his 70s.
Two patients being treated for the disease since the spate of illnesses began in the middle of July have died.
A second man has been arrested in connection with three explosions that took place in Staffordshire last night.
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A man has been arrested on suspicion of attempted murder after explosions at three homes.
Staffordshire Police said officers received reports of blasts at houses in the Stoke-on-Trent and Staffordshire Moorlands area last night.
A spokesman said:
A 44-year-old man from Cheadle was arrested on suspicion of attempted murder and is currently being interviewed by detectives.
A hot tub is thought to have been the probable source of an outbreak of Legionnaires' Disease, the Health Protection Agency has said.
Based at JTF Warehouse in City Road, Fenton, Stoke-on-Trent, samples taken from the hot tub were found to have an unusual strain of legionella bacteria, which matched that of the strain taken from patients with the illness.
Dr Sue Ibbotson, regional director of Health Protection Agency, West Midlands, said the organisation has taken histories from those with Legionnaires and 17 of the 18 confirmed cases visited the warehouse in the two weeks before they fell ill. One patient died after being affected by the disease.