Officials confirmed a 70-year-old man died Wednesday following the death of a 64-year-old woman Monday. Both are thought to have had underlying health problems, The Daily Telegraph reported.
The life-threatening disease, believed to have originated in cooling towers and large air conditioning units on one of a number of industrial sites, has infected 14 other people, officials said.
The cases are spread over a large area with seven local authorities involved in the investigation, and officials admitted the public remains at risk of infection until the source can be identified.
The councils involved in the investigation include Cardiff, Merthyr Tydfil, Rhondda Cynon Taf, Caerphilly, Blaenau Gwent, Torfaen and Monmouthshire.
Symptoms of the disease, which causes lung infections and pneumonia, include muscle ache, tiredness, headaches, dry cough and fever.
"Legionnaires' disease is a rare but potentially life-threatening illness," said Dr. Gwen Lowe, a consultant in communicable disease control for Public Health Wales.
"Most of the cases of Legionnaires' disease that are notified to us are isolated cases, but outbreaks can occur."