Researchers Andrew Rambaut of the University of Edinburgh, Oliver Pybus of Oxford University, and Yi Guan of the University of Hong Kong, say the virus originated from several viruses circulating in swine, and they believe the initial transmission to humans occurred several months before the outbreak was recognised.

The researchers say the movement of live pigs between Eurasia and North America appears to have facilitated the mixing of diverse swine influenza viruses, leading to the new H1N1 strain and that the closest related swine viruses existed up to 17 years ago and may have been circulating in pigs for several years before it's emergence in humans.

The U.S. Agriculture Department now says it will initiate a pilot surveillance project to look for new strains of flu virus in pigs.

Dr. Chan says it is important to get the right balance between complacency and vigilance and that pandemic strategies would vary between countries depending on their specific situation - the WHO does not recommend closure of borders or any restrictions on the movement of people, goods or services.

Some experts have warned that the consequences of the virus could be more serious in the developing world and poorer nations, especially those in the southern hemisphere now heading into their winter season, face the greatest risk from the flu pandemic - there is concern that the virus might mutate in the southern hemisphere over its winter and become more virulent.

Swine flu symptoms are usually similar to seasonal flu and the best protection is to exercise good personal hygiene, such as the washing hands thoroughly, covering the nose when sneezing and the immediate disposal of used handkerchiefs. People who are infected should stay home and avoid mixing with others.