An infected building cooling tower or rogue water system may have caused the outbreak, which has killed a woman of 59 and a man of 36 since 11 January.
A third victim has been interviewed by environmental health officers trying to find a link between the cases. She is expected to survive.
The three victims live within 10 miles of each other in the Cynon Valley near Merthyr.
Bro Taf Health Authority communicable diseases consultant Arun Mukerjee said it was extremely rare to find three cases in such a small area.
"We're extremely worried. We have lost two healthy people who need not have died," he said.
If officials decide a building cooling tower or water system is the most likely source of the outbreak, Health and Safety Executive officers can inspect building owners' maintenance records for possible breaches of health and safety regulations.