A third person has died of Legionnaires' disease, public health chiefs said today.

The 49-year-old woman died on Sunday evening following the death of two pensioners last week.

Only the most recent victim is linked to the outbreak in South Wales, which already has 18 other confirmed cases, Public Health Wales said.

The 70-year-old man and 64-year-old woman who both died of the disease have been excluded from the outbreak, along with five other people with the illness.

All patients linked to the outbreak of the disease have required hospital treatment.

Health officials continue to investigate the source and said the outbreak area is in the corridor 12km either side of the Heads of the Valleys Road between Abergavenny and Llandarcy.

Patients are linked to the outbreak if they live in or have visited the area in the two weeks before they fell ill.

Public Health Wales (PHW) is continuing to work closely with the Health and Safety Executive (HSE) and environmental health officers from seven South Wales local authorities.

Investigations are focusing on a cluster of seven people linked to Rhymney, and a further potential cluster of four people in the Cynon valley is also being investigated, PHW said.

The other eight people have various connections across the outbreak area.

A further case possibly linked to the outbreak is also being investigated.

Three industrial cooling towers have been closed for cleaning as a precaution, two in the Rhymney valley and one in Merthyr Tydfil.

None of the sites has been confirmed as the source of the outbreak.

The 10 registered cooling towers and evaporative condensers in the Merthyr Tydfil, Blaenau Gwent and Rhymney areas have been visited by officials trying to locate the source.

More than 100 separate workplaces have also been visited by the HSE in an effort to trace the source.

Dr Gwen Lowe, chairman of the outbreak control team at PHW, said: "Legionnaires' disease cannot be passed from person to person. We therefore investigate places people have been to, where they may have come into contact with a source of infection.

"On a precautionary basis, we are investigating people who live in, or have visited, a large geographical area over a long time period. However, this means that some of the cases we are investigating are likely to be sporadic with no links to other cases. It is also possible that there may be more than one source of infection.

"This is a very complex outbreak. Our investigations show that there is no single building visited by all the people linked to the outbreak. The sources, or source, are therefore likely to be industrial processes such as cooling towers."

Dr Lowe added: "A team of more than 100 staff from 10 agencies has been working to find the source of the outbreak. We will continue to do so until we are confident that we have found the source and removed it.

"Legionnaires' disease is a rare but potentially life-threatening illness. It has an incubation period of up to three weeks so we are likely to see further cases of illness even after the source is removed."