Scientists say predicting the timescale and impact of any flu pandemic is extremely difficult, but the United Nations World Health Organization has several "suggested assumptions" outlined in its pandemic guidance.

How long any pandemic lasts would be key to its economic impact. The world is already struggling with the greatest financial crisis since the Great Depression, straining budgets of governments, some of which are trying to cut spending.

* An influenza pandemic can occur when a new influenza virus subtype emerges and spreads easily among human beings.

* An influenza pandemic can begin at any time of the year and any place in the world, and is expected to spread to the rest of the world within several weeks or months.

* A single wave of pandemic could last several weeks to a few months, but will likely vary from country to country and even by community.

* Most communities are expected to experience multiple waves of a pandemic.

* Increased hospitalizations, deaths and other effects are expected to vary widely among countries and communities. Vulnerable populations are expected to be affected more severely.

* People will stay away from work not only because they are sick themselves, but because they are caring for sick relatives, taking care of children home from school, or simply lying low. Workplace absenteeism will be higher than the estimated clinical rate.