A total of 15 people are being treated in hospital after an outbreak of Legionnaires' Disease.
The Health Protection Agency (HPA) said all of those affected in Stoke-on-Trent, Staffordshire, are aged between their late 40s and mid-70s.
The number of confirmed cases has been steadily rising since news of the outbreak on Tuesday.
Two of the patients are in a critical condition and are being treated in separate hospitals.
A spokesman for University Hospital of North Staffordshire said: "The Trust is currently treating 14 patients confirmed with Legionnaires' Disease; 13 patients are in a stable or improving condition, one person is in a critical condition."
A spokesman for Glenfield Hospital, Leicester, said: "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."
Public and environmental health experts from the HPA, the Health and Safety Executive, the NHS in Stoke and Stoke-on-Trent City Council continue to work together to identify and investigate possible sources.
Professor Harsh Duggal, director of the Health Protection Agency's Health Protection Unit in Stafford, said: "Our investigation into possible sources continues.
"We have sampled and disinfected potential sources of exposure but people can develop symptoms up to two weeks after being exposed to legionella so we are not surprised by these new cases.
"We continue to map the outbreak and work with our partners to pinpoint possible sources, using the information we have gathered."
The HPA said the outbreak is not hospital-related and the hospital is a safe environment for visitors.
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 http://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.