In June 2009, The World Health Organisation (WHO) raised the level of alert to phase six. This means that the virus is spreading in at least two regions of the world, with rising cases being seen in the UK, Australia, Japan and Chile.
The move from level five to six doesn't mean that the symptoms of the virus are more serious or that it's causing more deaths.
What is swine flu?
Swine flu is a respiratory disease that normally only infects pigs. What's different about this virus is that it's a new version of the H1N1 strain of influenza type A.
This means it contains genes from pig, bird and human flu viruses.
Since the late 1950s human infections have been found in a small number of cases and usually in people who've had direct exposure to pigs.
In Europe, since 1958 a total of 17 cases have been reported. Secondary cases spread from human-to-human have been rare.
Where did it all begin?
It's unclear at this stage where the virus originated, although Mexico has seen the highest number of cases and it has since reached 74 countries.
There are a variety of reasons why so many people have died from the virus in Mexico.
It may be that people affected in this country have not sought treatment early enough, or that the virus has probably been circulating for several weeks, yet only showing mild symptoms in the majority of people.
Other health experts have suggested that nutritional and general living standards could have played a role and that the medical response wasn't as fast as it would be in other countries.
'In all cases outside of Mexico, the symptoms of this illness are mild and all patients have made a full recovery,' Alan Johnson, health secretary.
How is it spread?
Swine flu was initially spread by infected pigs, but has now spread between humans in the same way as normal flu though coughing and sneezing.
It's also spread through touching surfaces contaminated with the virus and then by touching the nose or mouth.
The seasonal flu jab is unlikely to stop the disease.
Leaflets about the swine flu outbreak have been delivered to every UK household, along with a TV and radio campaign.
I have flu-like symptoms. Do I need to be tested?
Much like the common flu virus, you will feel feverish with a high temperature.
You may have a sore throat, a headache, tiredness and muscle aches.
Some people experience diarrhoea and vomiting. In young children, warning signs include fast or troubled breathing, a bluish skin tone and irritability.
Speak to your doctor (GP) or call NHS Direct (0845 4647) for advice. In the meantime, stay at home, rest and drink plenty of fluids.
You're advised not to go to your doctor's surgery to avoid spreading the virus to others.
It's important that you contact a doctor if you've recently travelled to Mexico or another affected area and have two or more of the following symptoms:
- a fever of 38°c or more
- runny nose
- limb/joint pain
What can I do to protect myself?
Make sure you always cover your mouth and nose when you sneeze and use a tissue.
Don't leave tissues lying around where people can come into contact with them.
Avoid close contact with people who appear unwell and have a fever and cough. Wash your hands frequently with soap and water and keep surfaces and door handles clean.
There is no risk from eating pork.
What drugs are available to treat swine flu?
The government has enough antiviral drugs (such as Tamiflu and Relenza) to treat 50 million people if they become ill.
When treated with these drugs at an early stage, patients should respond well and fully recover.
What travel advice is there?
The Foreign Office is no longer advising against all but essential travel to Mexico. Check its website for advice on travelling to and from specific countries.
In the UK there are no travel restrictions, but people using public transport are being reminded to maintain good hygiene.
If you're returning from Mexico or any other affected countries and showing any flu-like symptoms, you should immediately contact your doctor or call NHS Direct.
What's the good news?
According to the European Centre for Disease Prevention and Control, it's still too early to know for certain whether the current situation will develop into a pandemic.
In the 20th century there have been three flu pandemics.
The Spanish flu outbreak in 1918 affected up to 40 per cent of the population with more than 50 million people dying.
The Asian flu pandemic in 1957 killed two million people.
The Hong Kong pandemic in 1968 killed up to one million people globally.
The H1N1 virus is not as lethal as bird flu – H5N1 - which health officials say is a much more dangerous virus because it can mutate rapidly.
We are also being reassured that the Government has prepared itself for a pandemic by stockpiling antiviral drugs and providing 35 million facemasks for NHS staff.
Health Protection Agency (HPA) scientists have revealed they have now worked out the full genetic code of the virus, which will help towards developing a vaccine.
The WHO judges Britain and France as the best-prepared nations for a pandemic.
Where can I get more information?
A pre-recorded swine flu helpline is available on 0800 1513 513 (or call 0044 207 928 1010 if you're overseas).