Swine flu has spread across the world since emerging in Mexico and is now officially the first flu pandemic for 40 years. Experts fear millions of people will be infected.

What is swine flu and what are the symptoms?

SWINE FLU SYMPTOMS Typical symptoms: sudden fever (38C or above) and sudden cough Other symptoms include: 1. Tiredness and chills 2. Headache, sore throat, runny nose and sneezing 3. Stomach upset, loss of appetite, diarrhoea 4. Aching muscles, limb or joint pain Source: NHS

Swine flu is a respiratory disease, caused by a strain of the influenza type A virus known as H1N1.

H1N1 is the same strain which causes seasonal outbreaks of flu in humans on a regular basis.

But this latest version is different: it contains genetic material that is typically found in strains of the virus that affect humans, birds and swine.

Although the strain may have originated in pigs, it is now a wholly human disease.

It can be spread from person to person by coughing and sneezing.

Symptoms of swine flu in humans appear to be similar to those produced by standard, seasonal flu - fever, cough, sore throat, body aches and chills. Some people with the virus have also reported nausea and diarrhoea.

However, many people who get flu show no symptoms at all.

Health experts say this could happen in half of all cases and with swine flu an analysis has shown that for children one in four of those infected may not fall ill.

What are the risks of the flu?

Experts have wrestled with the question ever since the pandemic emerged.

The problem is that for most people it is mild - about 98% recover without the need for any hospital treatment.

But doctors have found it very hard to predict who will develop complications, hence it has been dubbed a "Jekyll and Hyde" virus.

A fifth of the people who have died have been previously healthy individuals without any health conditions.

Nonetheless, certain groups are known to be at higher risk.

Pregnant women are between three to four times more likely to get seriously ill.

Young children also have higher rates of hospitalisation - although this could be partly because doctors are quicker to admit them.

Should I have the vaccine if offered it?

The vaccination programme is well under way.

Nearly 14m people, including NHS workers, people with health problems and pregnant women, were due to be offered the jab before Christmas 2009. Healthy children under the age of five were also due to start getting it.

But already polls have suggested people are worried about whether to get vaccinated.

The government says that getting immunised is the best way to protect yourself against flu.

However, its experts recognise that people are jumpy about vaccines and have said it is very much a personal decision.

People who are suspicious of vaccines often talk about the risk of complications.

One of the most common which is mentioned in association with flu vaccines is Guillain-Barré syndrome (GBS), a rare disorder in which a person's own immune system damages the nerve cells, causing muscle weakness and sometimes paralysis.

Most of the evidence suggests vaccines do not increase the risk, however there is research to indicate that getting flu increases it seven-fold.

Another allegation sometimes made is that being given the vaccine could actually give you flu.

Doctors are adamant this is not the case - the two vaccines being used in the UK do not contain the live virus.

As for deaths, the World Health Organization has been monitoring this and while a handful of people have died after being given the vaccine, officials have ruled out any link to the jab.

Why has the UK been more affected than other countries?

The UK has a comprehensive flu surveillance system, hence more cases may be identified in the UK than in other countries with less comprehensive monitoring services.

Also, the spread of the infection in the early stages was linked to flights out of Mexico.

FLU PANDEMICS 1918: The Spanish flu pandemic remains the most devastating outbreak of modern times. Caused by a form of the H1N1 strain of flu, it is estimated that up to 40% of the world's population were infected, and more than 50 million people died, with young adults particularly badly affected 1957: Asian flu killed two million people. Caused by a human form of the virus, H2N2, combining with a mutated strain found in wild ducks. The impact of the pandemic was minimised by rapid action by health authorities, who identified the virus, and made vaccine available speedily. The elderly were particularly vulnerable 1968: An outbreak first detected in Hong Kong, and caused by a strain known as H3N2, killed up to one million people globally, with those over 65 most likely to die

With Mexico being a popular tourist destination for British tourists, Britain was one of the first countries alongside the US and Canada to start seeing cases.

As the UK was affected earlier than some other countries the infection spread to a higher number of people at the start of the pandemic.

However, as the British summer progressed, cases fell, while southern hemisphere countries saw rises as they went through their winters.

More recently cases have started going up again in the UK, but other countries, such as France, also have high rates.

What should I do if I think I have it?

As the National Flu Service, which dealt with suspected cases at the height of pandemic has now closed, anyone who suspects they have swine flu should ring their GP practice for advice.

The flu information website is still available at www.direct.gov.uk/pandemicflu.

In the initial phase of the outbreak, lab testing was done to diagnose the flu but this is no longer happening routinely.

How is swine flu treated?

Two drugs commonly used to treat flu, Tamiflu and Relenza, are effective at treating infection, reducing the length of the illness and cutting the chances that people will have serious complications.

Use of these drugs may also make it less likely that infected people will pass the virus on to others.

However, the drugs must be administered at an early stage to be effective.

Researchers have also questioned whether the drugs are helpful in children aged between one and 12, saying the risk of side effects might outweigh any benefits.

But the Department of Health maintained a "safety-first approach" of offering antivirals to everyone remains a sensible and responsible way forward.

It said it would keep the policy under review.

Where can I get further advice?

Further information and advice on swine flu can be found at websites of leading health and research organisations around the world. The World Health Organisation gives background information on the virus.

The UK's government services website is carrying regularly updated health and travel information. The Health Protection Agency advises the public about what to do if returning from an affected area. NHS Choices outlines how swine flu is different from other flu.

The European Centre for Disease Prevention and Control is another good source of information.

The US government's Centers for Disease Control and Prevention is counting the number of cases in the US.

You can also track spread of swine flu reports using unofficial sources. Google is mapping search term data as an indicator of flu activity both across the US down to state level and in Mexico. Healthmaps maps viruses using news reports. Social media guide Mashable lists a range of ways to track the virus .

Information and links to useful websites are being shared on Twitter, the micro-blogging service, while social networking website Facebook is tracking swine flu discussion amongst users.

Further questions from our readers have been answered by a UK-based expert.

And the BBC's medical correspondent, Fergus Walsh, is filing regular entries on his blog on H1N1.