Public warned that several physical conditions contribute to severe cases of the disease

The number of swine flu cases in most countries around the world continues to mount and young people are the ones most affected, according to the World Health Organisation (WHO). It said the average age of those affected, based on information from Canada, Chile, Japan, UK and the US, is between 12 and 17 years of age.

Since the pandemic remains in its relatively early stage of development this fact could reflect that the earliest cases often occur as school outbreaks. But as the disease expands into the wider community, the average age of the cases is beginning to increase slightly, said the WHO.

‘Our understanding of the disease continues to evolve as new countries become affected, as community-level spread extends in already affected countries, and as information is shared globally,’ reported the health body.

It remains one of the organisation's top priorities to determine which groups of people are at highest risk of serious disease so steps to best protect them can be taken.

Several existing diseases are considered risk factors for developing a serious case of swine flu. These include, asthma and other respiratory diseases, cardiovascular disease, diabetes and cancer. Pregnant women are also at higher risk of developing a more severe form of the disease.

Recent medical reports suggest that obese people and some minority groups may also have an elevated risk of developing complications. But this is not yet completely clear.

Manufacturers are expected to have swine flu vaccines available for use around September time.