INFLUENZA experts have warned Australians against hosting "swine flu parties" after reports of Americans trying to get the virus while it was mild to build immunity.

According to reports, parties are being held in the US where healthy children interact with sick ones, aiming to get the virus and produce antibodies against it in case a more severe second wave develops.

One Melbourne parent told The Age of hearing people talking about the concept locally, but did not know anyone who had acted on it.

Anne Kelso, director of the World Health Organisation's Influenza Centre in Melbourne, said she had heard of such parties, but did not approve of them. She said flu viruses could cause severe illness and had killed healthy people in the past. "It's a balance of risk I would not encourage."

Alan Hampson, chairman of Australia's Influenza Specialist Group, also discouraged the idea because it could hasten the spread of the virus, putting people with existing health conditions at serious risk.