Two people were taken to the University of Wales Hospital in Cardiff and the other cases were admitted to East Glamorgan Hospital, near Pontypridd.

Two of the patients involved have now returned home.

None of the illnesses are thought to be related and investigations are under way to determine if there is a common source.

BBC Wales's Gavin Thomas reports on the south Wales cases

Dr Arun Mukerjee, a consultant in communicable diseases at Bro Taf Health Authority, said no connection between the cases had been detected.

"We are doing a very wide ranging investigation to establish the source of this infection," he said.

The legionella bacteria lives and grows in water and is transmitted by water droplets or mist. It is not contagious or infectious.

Respiratory infections expert Dr John McFarlane, from Nottingham City Hospital, said incidents of legionnaires' disease are more common in the summer months.

Some cases can be passed off by GPs or hospitals as pneumonia without being correctly diagnosed.

Dr McFarlane said it was possible that a common source existed with the cases in south Wales, although three quarters of all recorded incidents occur sporadically without an obvious source.