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
Human body with internal organs
Typical symptoms: sudden fever (38C or above) and sudden cough
1. Other symptoms include: 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
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 of H1N1 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.
A fever - which is a temperature of 38ºC (100.4ºF) - is the key symptom, combined with other complaints which may include a cough, sore throat, body aches, chills and aching limbs. Some people with the virus have also reported nausea and diarrhoea.
As with normal flu, the severity of symptoms will depend on treatment and the individual. Many people have only suffered mildly and have begun to recover within a week.
People are most infectious soon after they develop symptoms, but they cease to be a risk once those symptoms have disappeared. The incubation period may be as little as two days.
Apparently healthy people are dying from the virus. Does that means it is getting worse?
Experts say this does not change anything.
THE AT RISK GROUPS
People with lung disease
People with heart disease
People with kidney disease
People with diabetes
Those with immunosuppression problems either because of treatment or disease
Patients who have had drug treatment for asthma
Children under five
Apparently healthy people can die of any flu-related virus if it causes complications such as pneumonia so these latest deaths do not give any extra cause for concern.
Indeed, so far, many people who have developed symptoms of infection have not needed drugs to make a full recovery, according to the WHO.
Flu expert Professor Peter Openshaw, of Imperial College London, says about one in every three people who become infected will not realise they have had swine flu because they will have had no or only very few symptoms.
"About 98% of people who get infected will recover fully without any hospital treatment so I think the public needs to be reassured."
The real fear is that the strain will mutate and become more virulent which would pose a greater threat. This has been the feature of previous flu pandemics.
But this has not yet happened - and in any event it is worth remembering that seasonal flu often poses a serious threat to public health - each year it kills 250,000 - 500,000 around the world.
Why are younger people more affected?
In the UK the infection has been found to affect younger people more due to it spreading quickly in schools where there is a lot of mixing and where infections can quickly spread. This is not unusual and children are known to be important in the spread of flu. It is thought that older age groups may have some immunity as they have been exposed to previous H1N1 viruses.
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. With Mexico being a popular tourist destination for British tourists, Britain was one of the first countries alongside America and Canada to start seeing cases.
As the UK was affected earlier than some other countries the infection has spread to a higher number of people.
What should I do if I think I have it?
Anyone with flu-like symptoms who suspects they might have the swine flu virus are being advised to stay at home and contact the National Flu Service on 0800 1 513 100 or via the internet at www.direct.gov.uk/pandemicflu
The service allows sufferers to get access to anti-flu drugs without the need to consult a GP.
However, those with underlying health conditions, the under ones and pregnant women are still being advised to contact a doctor.
The GP route is also open to anyone who does not want to use the service.
In the initial phase of the outbreak, lab testing was done to diagnose the flu but this is no longer happening routinely.
How is it 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 maintains 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.
The UK government already has a stockpile of Tamiflu, ordered as a precaution against a pandemic.
There is also concern that if too many people start taking anti-virals as a precaution, it could raise the risk of the virus developing resistance, reducing the drugs' effectiveness.
There is however no evidence at present that this is happening.
In any event there is little point taking these drugs as a precaution as each tablet only provides a day's worth of cover. Given that the virus may be with us for many months - or indeed years - taking a regular pill is ill-advised as the long-term side effects are not known.
The drugs can have side-effects, most commonly nausea and vomiting.
What measures then can I take to prevent infection?
As yet there is no licensed vaccine, but manufacturers are trying to develop one. Good progress is being made and it is hoped the first doses will be available by mid-October.
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
The government has announced that the over 65s and those with underlying health conditions - basically those currently eligible for the seasonal flu vaccine - will be immunised first.
Pregnant women will also be offered the vaccine as a priority.
It is hoped that even if the virus mutates in coming months, the vaccine would still confer a high degree of protection against related strains.
In the meantime, avoid close contact with people who appear unwell and who have fever and cough.
General infection control practices and good hygiene can help to reduce transmission of all viruses, including the human swine influenza.
This includes covering your nose and mouth when coughing or sneezing, using a tissue when possible and disposing of it promptly.
It is also important to wash your hands frequently with soap and water to reduce the spread of the virus from your hands to face or to other people, and cleaning hard surfaces like door handles frequently using a normal cleaning product.
In Mexico masks have been handed out to the general public, but experts are sceptical about how useful this is.
Some suggest it may even be counterproductive.
What is the advice for pregnant women?
The Department of Health has clarified its advice on how expectant mothers should protect themselves following a series of apparently mixed messages.
Concerns were heightened after a woman with swine flu died last week shortly after giving birth prematurely.
Pregnant women are among the groups at increased risk from swine flu. It is important that they follow the advice about hand hygiene.
They may also want to avoid very crowded places and unnecessary travel, but experts stressed people should use their own judgement and should carry on with their daily lives.
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.