Governments worldwide have been on high alert since a new human flu virus emerged in April 2009 in Mexico and the United States. The World Health Organisation (WHO) declared a global pandemic in June and warned that the new A(H1N1) strain - also called swine flu because it may have originated in pigs - could infect hundreds of millions of people.

Swine flu is genetically different from the A(H1N1) virus that causes normal seasonal flu and, because it is new, most people have little or no immunity to it.

The virus is spread as easily as seasonal flu. When an infected person sneezes or coughs, tiny droplets are dispersed through the air and can be inhaled or contaminate surfaces.

The virus enters the body through the nose or throat. Symptoms include a fever, cough, headache, muscle and joint pain, sore throat and runny nose, and sometimes vomiting and diarrhoea.

Unlike seasonal flu, the new virus affects younger people the most. Although the virus can be fatal, most people so far have recovered within a week. Worryingly, there are reports of a very severe form of the disease which infects the lungs of young and otherwise healthy people, causing major respiratory failure and death.

Anti-viral drugs oseltamivir (known as Tamiflu) and zanamivir are used to treat more severe cases, and drug companies are producing a vaccine to offer some protection from the virus.

Health experts are concerned the virus may have a devastating impact in many parts of the developing world because of people's lack of access to health care.

Common seasonal flu kills between 250,000 and 500,000 people in an average year. Most deaths in industrialised countries occur among people over 65 years old, but little is known about the effects of seasonal flu in developing countries.

There have been three major pandemics in the last century, caused by large genetic changes in the flu virus. The most recent was in 1968-69, when the "Hong Kong" pandemic killed about one million people globally. In 1957-58 the "Asian flu" killed more than a million people, and in 1918-19 the "Spanish flu" pandemic killed at least 40 million people - most of them young healthy adults.

The WHO says the world is now better prepared to withstand a flu pandemic.