Q. How infectious is swine flu?

A. In the days since the disease was first reported in Mexico and the US it has spread across the world, triggering the World Health Organisation's decision to raise the pandemic threat from level 3 to level 4 (out of six). Flu is one of the most infectious illnesses known and the WHO has already recognised that the outbreak cannot be contained.

Q. Will we see it spread like ordinary winter seasonal flu?

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A. That is not yet clear. Outside Mexico, swine flu has not so far been shown to spread widely in the community. Outbreaks appear to have been caused by travellers returning from Mexico spreading the virus in schools or to their families and close contacts. The first confirmed cases in Britain, Iain and Dawn Askham of Falkirk, Scotland, who had been on honeymoon in Cancun, Mexico, had contact with 22 people in the week since they returned, of whom nine have developed symptoms that could be swine flu.

Q. How bad is it going to get?

A. So far, cases outside Mexico have been mild. No one knows how it is going to develop but the WHO has warned that it would be a mistake to be lulled into a false sense of security. The 1918 Spanish flu pandemic started mildly – and went on to kill 20 to 40 million people round the world. "I think we have to be mindful and respectful of the fact that influenza moves in ways we cannot predict," Keiji Fukuda, WHO acting assistant director-general, said.

Q. Could it die away over the summer?

A. Yes – but only temporarily. Swine flu has struck at the end of the winter flu season in the northern hemisphere. Flu normally dies away in summer, only to return the following winter. The reasons are thought to be linked with the increase in ultraviolet light from longer days, which destroys the virus, and the fact that people spend less time huddled together indoors where it is easier to transmit the infection. The worry is that if it subsides over the summer it could return with a vengeance next winter, in a more virulent form. Professor Neil Ferguson, of Imperial College, London, said yesterday that up to 40 per cent of the population in Britain could be infected.

Q. Can I call the national flu line for advice?

A. Not before next autumn. Lord Darzi, health minister, said the dedicated telephone and web based advice service, which would be a key feature of the countries defence against a pandemic helping patients diagnose their symptoms and obtain anti-viral drugs, would not be up and running for six months. The service is supposed to be capable of taking six million calls a week and come on line when the WHO raises the threat level to Phase 5 (it is now Phase 4).

Q. Is it worth taking Vitamin C?

A. Alternative health practitioners of homeopathy, herbal remedies and nutritional medicine are recommending measures to protect against the flu. They are about as useful as a water pistol against a forest fire.

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Q Do masks provide protection against swine flu?

A Yes, but probably not to the person wearing them. They help stop the virus being expelled from the mouth and nose when coughing or sneezing, but are much less effective at protecting the wearer from a virus picked up on the hands or circulating in the air. Wearing a mask thus becomes a public-spirited act.

Q Now swine flu has arrived in Britain how likely is it to spread?

A Highly likely – if it hasn't already. There were 14 suspected cases under investigation in Britain yesterday. It is a new virus, with a combination of genes from pigs, birds and humans, and has proved that it is easily transmitted from human to human. As it is a novel virus, immunity to it is likely to be low and almost anyone could be vulnerable.

Q Should Britain screen travellers at its borders?

A Flu is one of the most infectious illnesses known – one cough by an infected person can transmit the virus to a roomful of people. The disease takes up to seven days to produce symptoms so most travellers would not know they were infected until after they reached their destinations. Screening at airports and land borders may pick up some travellers but the effect would be limited. Politicians like to impose border controls because it shows they are doing something.

Q Will this outbreak develop into a pandemic?

A It has all the hallmarks of one and has already been identified on both sides of the Atlantic. Just how rapidly it is spreading will be confirmed in the coming days. What we don't know is how virulent the virus is. While some victims have died, others have recovered after only a mild bout of illness.

Q What defence do we have against swine flu?

A Better than we did against the last pandemics in 1957 and 1968. We have a stockpile of anti-viral drugs – Tamiflu and Relenza – which we did not have then. We also have a pandemic plan, drawn up by the Government since avian flu became a threat in 2003, which sets out what is to be done – from distributing the drugs and setting up helplines to closing schools and banning public events.

Q Has the pandemic plan ever been tested?

A Yes, in one of the biggest emergency planning exercises since the end of the Cold War that took place in 2007. It involved hundreds of health officials across the country.

Q Are there enough anti-viral drugs?

A Not according to the Tories. The Government says it has over 30 million courses of the drugs, enough for half the population. The Tories say this is not enough if family members of an infected person are to be treated prophylactically. In that case, enough drugs to cover three-quarters of the population will be necessary, they say.

Q How do anti-viral drugs work?

A Two anti-viral drugs have been licensed in Britain in the last decade: Tamiflu and Relenza. The problem with the flu virus is that it is constantly mutating, so a new vaccine has to be produced each year. The anti-viral drugs get round this by targeting not the virus itself but an enzyme that enables the virus to spread from cell to cell. Provided they are taken within 48 hours of the onset of symptoms they can shorten the illness and reduce its severity.

Q What is the difference between Tamiflu and Relenza?

A The big difference is that Tamiflu is taken as a pill while Relenza is inhaled directly into the lungs.

Q Why has the Government got both drugs in its stockpile?

A Because resistance has emerged in flu strains in some countries, rendering the drugs ineffective.

Q What about a vaccine?

A Pandemics tend to happen in waves separated by several months, so a vaccine could potentially be developed against the second or third wave, if they occur. A vaccine first has to be matched to the exact strain of the virus and then incubated in hen's eggs, which may take up to six months. Hundreds of millions of doses would be required which would put huge pressure on the world's laboratories.

Q Shouldn't a vaccine be the first priority?

A Previous vaccines against swine flu have turned out to be worse than the disease. An outbreak in the US in 1976 infected 200 people, of whom 12 were hospitalised and one died. But before it was over 40 million people had been vaccinated, 25 of whom died and 500 of whom developed Guillain-Barre syndrome, which can be fatal.

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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?

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.