West Australian health authorities are desperately trying to find the cause of an illness that has killed four young children in Perth.

The WA Health Department today announced a fourth child had died from a similar cause of illness as three who died two weeks ago.

The three children who died earlier this month all had a combination of influenza A and a bacterial infection.

Paul Van Buynder, director of communicable diseases at the West Australian health department, said the latest child did not have the flu but had contracted pneumococcal bacteria.

"There isn't a single bacterial infection that links the four cases,'' Dr Van Buynder said.

"At this point we are unable to say what the cause of death has been in these four children.''

Dr Van Buynder said it was "very uncommon'' for children to die with an infectious disease so quickly, but he stressed Perth did not have a epidemic on its hands.

"We are desperately trying to get to the bottom of what's causing these illnesses,'' he said.

Health department officials held a teleconference with health experts from around the country today to discuss the issue and have sent bacterial specimens from the four children interstate for further testing.

David Smith, director of microbiology at Pathwest, WA's pathology laboratory, said it appeared the four children had a bacterial infection in the respiratory tract.

"It appears more that in these infections the bacteria either got very quickly into the blood without causing a real pneumonia, that would have given the parents more warning, or that they produced a toxin that got into the system and affected them,'' Mr Smith said.

"And that does mean that the course of the illness was quicker than we normally see with these bacterial infections.''

Parents whose children have common winter colds and coughs are being told not to be alarmed but to seek medical advice if their children develop a high fever, become listless and have a cough.