Swine flu is an acute respiratory disease which affects pigs. In pigs, according to the World Health Organisation, "morbidity tends to be high and mortality low" - which means that it spreads quickly but kills between 1 and 4 per cent of its victims.

Pigs are the perfect mixing vessels for what is known as reassortment of flu strains, when genetic material from swine flu, human flu and avian flu is jumbled up to create entirely new influenza strains to which humans have little or no immunity.

The currrent H1N1 virus contains genetic elements from North American swine flu, North American avian (bird) flu, and human and swine flu strains normally found in Asia and Europe. According to the Centres for Disease Control it is "an unusually mongrelised mix of genetic sequences".

This new strain of swine flu is not infecting pigs - and has never been seen in pigs.

Seasonal flu viruses (which mutate every year) kill between 250,000 and 500,000 people a year.

The symptoms produced by the current strain of swine flu resemble those of seasonal flu - fever, coughing, muscle aches and extreme tiredness - but it also appears to cause diarrhoea.

The currrent H1N1 virus contains genetic elements from North American swine flu, North American avian (bird) flu, and human and swine flu strains normally found in Asia and Europe. According to the Centres for Disease Control it is "an unusually mongrelised mix of genetic sequences".

The most lethal flu pandemic of the past century was also caused by a swine flu strain. One billion people are thought to have contracted "Spanish flu" in 1918-19, of whom around 50 million were killed - although the death toll could have been much higher.

In 1976, an Army recruit at Ford Dix, New Jersey, complained that he was feeling tired and weak. He died the following day. After Swine flu was diagnosed panicked officials persuaded Gerald Ford that the entire population needed vaccination. About 40 million people were vaccinated before another fear took hold - that the vaccine was more dangerous than the disease - and the programme was aborted.

The WHO's pandemic alert level has been raised a notch to level 4. Level 5 is considered a pandemic - with "sustained community-level transmission" in at least two countries - and level 6 a full-scale global pandemic affecting more than one region in the world.

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• Swine flu

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