Q: What is swine flu?

A: Much the same as human flu – but in pigs. The worry is that pigs are excellent hosts for the virus. And because they are genetically close to humans, they can pass the virus to us more easily than birds can. The great fear over the past decade has been that the avian flu virus, H5N1, would infect pigs which would act as a reservoir for its transmission to humans. Luckily for the world, apart from a few isolated outbreaks, this did not happen.

Q: How worried should we be?

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A: At this stage, no one knows. The virus that has caused the outbreak is a strain of the H1N1 type that contains bird, pig and human genes in a combination never seen before. Immunity to it will thus be limited. Scientists are working to establish the precise nature of the virus, the symptoms it causes and its capacity to cause disease and death.

Q Has swine flu infected humans before?

A: Yes. There have been rare cases since the 1950s, mostly in people such as farmers who work directly with pigs. In Europe, 17 cases have been reported since 1958. In the US, an outbreak at a military camp in New Jersey in 1976, infected over 200 soldiers, of which 12 were hospitalised and one died.

Q What are the symptoms of swine flu?

A: Similar to ordinary human flu – cough, sudden fever, headache, muscle pains. In severe cases, it may lead to pneumonia, multi-organ failure, and death. The incubation period for ordinary human flu is two to five days.

Q: Can it be treated?

A: Yes – up to a point. Early indications are that patients in Mexico and the US have been successfully treated with the antiviral drugs Tamiflu and Relenza. These drugs cannot prevent flu but they can limit its severity, and thus save lives, if taken as soon as symptoms develop. However, the swine flu has proved resistant to older anti-virals such as amantadine.

Q: Is Britain prepared for a pandemic?

A: Better than it was five years ago. A pandemic plan has been prepared detailing action by everyone from pharmacists, who will hand out anti-viral drugs, to hospitals handling the seriously ill, to mortuaries which may have to be temporarily expanded. Over 14 million courses of Tamiflu have been stockpiled and the Government announced last year it was doubling the amount to provide enough for half the population.

Q: How bad might a pandemic be?

A: At its worst, it could have a devastating global impact, greater than a terrorist attack, nuclear accident or environmental disaster. The World Health Organisation estimates that a mild pandemic could cause up to 7.5 million deaths. In the UK, Sir Liam Donaldson, the Government's Chief Medical Officer, has said that in the worst case scenario the country could face up to 750,000 deaths. However, in the flu pandemics of 1957 and 1968 most victims recovered. There was no panic, cities did not empty, travel did not come to a halt and economies weren't devastated. Each of those pandemics killed 50,000 people in the UK and around one million worldwide. In a normal year, flu kills 12,000 to 20,000 mainly elderly people in Britain and 250,000 around the world.

Q: Who is at greatest risk?

A: In Mexico, the virus appears to be targeting those aged 20 to 40. This is not unusual – the same occurred during the worst pandemic of the last century, in 1918, when 20 to 40 million people died. Young healthy people with strong immune systems react most powerfully to the virus but the very strength of their reaction produces inflammation and secretions in the lungs which can be overwhelming. In the US, the virus appears to be targeting children who are suffering only mild illness. The difference in the two countries is so far unexplained. One hypothesis is that a second virus may be circulating in Mexico which is interacting with the swine flu virus to produce more severe symptoms.

Q: How can I protect my family?

A: By acquiring a stock of anti-viral drugs such as Tamiflu or Relenza, available only on prescription at an NHS cost of around £20 for a course of 10 doses (enough for one person). Otherwise, the best defence is strict personal hygiene. It is hard to better the advice printed by the 'News Of the World' on 3 November 1918: "Wash inside nose with soap and water night and morning; force yourself to sneeze night and morning, then breathe deeply. Do not wear a muffler, take sharp walks regularly and walk home from work; eat plenty of porridge." Porridge is, of course, a known cure-all – but the rest of the advice holds as true today as it did then.

Q: Is there a vaccine against it?

A: Not in humans (there is in pigs). Ordinary seasonal flu vaccine for humans might offer some protection because there are similarities between the H1N1 human flu viruses and the new H1N1 pig flu virus. Investigations are under way to see if the seasonal vaccine would have a protective effect but those will "take some time".

Q: Why has this outbreak started in Mexico and the US?

A: No one knows, but it is certainly a surprise. The next threat was expected to come from the Far East. Avian flu has spread through poultry populations, and 400 humans have been infected, 250 of whom have died. Health experts warned that a small mutation to the virus could turn a rare but lethal disease into one which could threaten the entire planet. Now, the threat has emerged – but on the other side of the world.

Q: Is it safe to eat pork?

A: Yes. Cooking destroys the virus.