About 40 percent of people who have died of swine flu or fallen seriously ill were young and otherwise healthy, an expert from the World Health Organisation told a conference in Vienna Monday.

Forty percent of the most serious cases, as well as deaths, concerned people "who would otherwise be considered healthy," WHO expert Sin Lun Tam told a conference on pulmonary diseases that runs until Wednesday in the Austrian capital.

Of those who had fallen gravely ill from the H1N1 virus, over 50 percent of were below the age of 20, he said.

Meanwhile, the death rate was highest among people between the age of 25 and 49, he added.

The expert also noted that between 15 and 30 percent of those admitted to hospital with the H1N1 virus had to be treated in intensive care.

And the origins of the disease were still unknown, he said.

In Australia and the United States, children made up the highest number of hospitalisations, according to the WHO.

At least 3,205 people have died of the disease since it was uncovered in April, the organisation said on Friday.