FACTBOX: Some facts about pandemic flu from the WHO

(Reuters) - The U.N. World Health Organization has said it is closely monitoring an outbreak of a new deadly strain of swine flu in Mexico and the United States.

The human-to-human spread of the virus has raised fears of a flu pandemic. The WHO has said it needs more information before it changes its pandemic alert level, currently at three on a scale of one to six.

Here are some facts about pandemic influenza from the WHO's website http:/www.who.int/csr/disease/influenza/pandemic10things/en/

* Flu pandemics are caused by new flu viruses that adapt into strains that become contagious between humans.

* Flu pandemics occurred three times in the past century: the Hong Kong flu in 1968, the Asian flu in 1957 and the Spanish flu of 1918.

* Experts agree that another pandemic could come at any time and could involve any one of a number of new strains of flu. Most eyes have been on the H5N1 strain of avian flu that first infected people in Hong Kong in 1997. Since 2003 it has infected 421 people in 15 countries and killed 257 of them.

* To be declared a pandemic strain, a virus must be new to humans, cause serious illness, be easily transmitted from one person to another, and sustain that transmission efficiently.

* All countries will be affected once a fully contagious pandemic virus emerges. Previous pandemics circled the globe in six to nine months, even before international air travel was common. Today, a virus could reach all continents in weeks.

* Widespread illness will occur. Most people will have no immunity to the pandemic virus. A substantial percentage of the world's population will need medical care, but few countries have enough staff, facilities, equipment and hospital beds.

* Supplies of vaccines and antiviral drugs will be inadequate. Many developing countries will have no access to vaccines.

* Many people will die. Based on the 1957 pandemic, the WHO conservatively estimates 2 million to 7.4 million people may die during the next outbreak, but says all estimates are speculative, and will depend on how many people become infected and the virulence of the virus, among other factors.

* Economic and social disruption will be great. High rates of worker absenteeism will impair services like power, transportation and communications.

* Every country must be prepared. The WHO has issued a series of recommended responses to the pandemic threat.

* The WHO will alert the world when the pandemic threat increases, working closely with governments and public health organizations on surveillance. WHO experts say it is too soon now to declare another pandemic.

(Reporting by Roberta Rampton; Editing by Maggie Fox and Will Dunham)