Seven people are being treated in hospital for Legionnaires' disease after an outbreak in a city.
The Health Protection Agency (HPA) said the patients are being treated at the University Hospital of North Staffordshire, in Stoke-on-Trent.
A spokeswoman for the HPA said one of those being treated is female and the age of the patients varies from late 40s to 75 years old.
"Two men in their seventies remain stable, the remaining patients are improving," the spokeswoman said.
"The HPA is also investigating two cases identified in early summer as being possibly linked to the current cluster."
Professor Harsh Duggal, director of the Health Protection Unit in Stafford, said an investigation was being carried out to try to identify the source of the outbreak.
He said: "While we do not currently have a direct link between the cases the evidence points to the fact that there is a common source.
"We are taking detailed histories of the movements of the patients to see if there are similar patterns which would indicate a local source of infection.
"Legionnaires' disease is a rare but potentially life-threatening illness.
"It is caused by a bacteria commonly associated with water systems and cannot be passed from person to person.
"As a precaution we are working with the Health and Safety Executive and Stoke-on-Trent City Council Environmental Health Services to identify and control any possible sources of the disease."
Public and environmental health experts from the HPA, the Health and Safety Executive, the NHS in Stoke and Stoke-on-Trent City Council are jointly investigating the seven confirmed cases.
The HPA said action taken so far includes identifying, sampling and advising on the disinfection of potential sources of the disease, such as cooling towers, in areas that those being treated may have visited in common.
Health care staff in the areas where the patients live, including GPs, have been alerted to the outbreak and to look out for patients who may be developing symptoms of the disease.
People have also been made aware of the symptoms of Legionnaires' disease, the HPA added.
The outbreak in Stoke-on-Trent is the second in the UK in as many months.
An outbreak in Edinburgh came to light at the end of May and is thought to have centred on a cluster of cooling towers in the south-west of the city.
Three people died after contracting the illness and last week the total number of confirmed and suspected cases stood at 101.
The outbreak in Scotland has since been declared over by health officials.
Early symptoms of Legionnaires' disease, which cannot be spread from person to person, include a flu-like illness with muscle aches, tiredness, headaches, dry cough and fever which can then lead to pneumonia, the HPA said.
It is caused by a germ called Legionella pneumophila.
As with any pneumonia, the patient can become very unwell. Diarrhoea and/or confusion may occur, as well as chest and breathing symptoms.
It is effectively treated with a course of antibiotics.
Although this bacteria is widely distributed in the environment it can lead to human illness if sources such as wet air conditioning systems are not well maintained.
People are advised if they are feeling unwell with any of the possible signs 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.