One of the people being treated for Legionnaires' disease in Stoke-on-Trent, Staffordshire has died today, the Health Protection Agency said.
The victim is among the 15 people had been affected by the disease. All of those affected are aged between 40s and mid-70s.
The number of confirmed cases has been steadily rising since news of the outbreak on Tuesday.
Public and environmental health experts from the HPA, the Health and Safety Executive, the NHS in Stoke and Stoke-on-Trent City Council are investigating the possible source of the outbreak.
The HPA said the outbreak is not hospital-related and the hospital is a safe environment for visitors.
A spokesman for University Hospital of North Staffordshire said: "The trust is currently treating 11 patients confirmed with Legionnaires' disease who are all in a stable or improving condition.
"One patient has sadly died.
"Three patients are well enough to go home and have been discharged."
A spokesman for Glenfield Hospital, Leicester, added: "We are treating a patient transferred from the University Hospital of North Staffordshire confirmed with Legionnaires' disease who is in a critical but stable condition."
Anyone feeling unwell with any of the possible signs is advised to go and see their doctor, ring NHS Direct on 0845 4647, or visit www.nhs.uk.
The HPA stressed that there is no need for anyone who is well or only mildly unwell to see their doctor or to have any tests.