Four cases of the disease, a severe form of pneumonia, have been confirmed in the county in the past five weeks and health officials have not ruled out the possibility of more cases.

The two other confirmed cases, who are also believed to be elderly people, are in a serious condition in hospital.

Tests are being conducted on a further two people who are suspected of having the disease.

Mike Deakin, the county's director of public health, said on Friday that the cases came from the Hereford city area.

There is nothing to say we are not going to get any more cases over the next few days. Mike Deakin, Herefordshire Director of Public Health

He said public health officials are urgently trying to establish the source of the outbreak to bring the infection under control and GPs throughout the county have been alerted.

Mr Deakin said: "The current situation is that we have had four cases in Hereford city and around it.

"Two of them have unfortunately died and two of them are quite seriously ill."

Mr Deakin said that under normal circumstances the county would expect to see an average of one case every two years.

He said: "That is why we are particularly concerned by this.

"There is nothing to say we are not going to get any more cases over the next few days.

"The incubation period is normally between two and 10 days but can be up to three weeks."

Most likely source

Environmental health teams are conducting thorough inspections of all "installations" that could be responsible for the spread of the disease including water cooling towers and water filtration systems, Mr Deakin explained.

But he said that at present no obvious link had been found between the four confirmed cases, although the evidence suggested the source of the outbreak was within the Hereford area.

He said: "Hereford does seem to be the most likely source."

The disease is caused by a bacteria called Legionella pneumophilia.

Although it is widely distributed in the environment and commonly lives in hot and cold water, and air conditioning systems, it is normally only found in very small numbers.

Contaminated particles

But when it comes into contact with warm water it multiplies quickly and can spread through the air in very small droplets.

Inhaling contaminated airborne particles is the main route of transmission for the disease.

Early symptoms of the disease include a "flu-like" illness with muscle aches, tiredness, headache, dry cough and fever.

Death occurs in up to 15% of previously healthy people who contract the disease.