Swine flu is a respiratory disease, caused by influenza type A which infects pigs.
There are many types, and the infection is constantly changing.
Until now it has not normally infected humans, but the latest form clearly does, and can be spread from person to person – probably through coughing and sneezing.
What is new about this type of swine flu?
The World Health Organization has confirmed that at least some of the human cases are a never-before-seen version of the H1N1 strain of influenza type A.
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.
Flu viruses have the ability to swap genetic components with each other, and it seems likely that the new version of H1N1 resulted from a mixing of different versions of the virus, which may usually affect different species, in the same animal host.
Pigs provide an excellent ‘melting pot’ for these viruses to mix and match with each other.
How dangerous is it?
Symptoms of swine flu in humans appear to be similar to those produced by standard, seasonal flu.
These include fever, cough, sore throat, body aches, chills and fatigue.
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.
So far, most cases of swine flu around the world appear to be mild, albeit with diarrhoea more common than is found with seasonal flu.
But lives have been lost in Mexico, and a single death – of a Mexican child – has been confirmed in the US.
How worried should people be?
When any new strain of flu emerges that acquires the ability to pass from person to person, it is monitored very closely in case it has the potential to spark a global epidemic, or pandemic.
The World Health Organization has warned that swine flu could potentially trigger a global pandemic, and stress that the situation is serious.
However, experts say it is still too early to accurately assess the situation fully.
Currently, they say the world is closer to a flu pandemic than at any point since 1968 – upgrading the threat from four to five on a six-point scale following a meeting on Wednesday.
Nobody knows the full potential impact of a pandemic, but experts have warned that it could cost millions of lives worldwide. The Spanish flu pandemic, which began in 1918, and was also caused by an H1N1 strain, killed millions of people.
There is hope that, as humans are often exposed to forms of H1N1 through seasonal flu, our immune systems may have something of a head start in fighting infection.
However, the fact that many of the victims are young does point to something unusual. Normal, seasonal flu tends to affect the elderly disproportionately
Can it be treated?
The US authorities say that two drugs commonly used to treat flu, Tamiflu and Relenza, seem to be effective at treating cases that have occurred there so far. However, the drugs must be administered at an early stage to be effective.
Use of these drugs may also make it less likely that infected people will pass the virus on to others.
The UK Government already has a stockpile of Tamiflu, ordered as a precaution against a pandemic.
It is unclear how effective currently available flu vaccines would be at offering protection against the new strain, as it is genetically distinct from other flu strains.
A new bespoke vaccine is being worked on by scientists in the UK and the US, but it may take months to perfect it, and manufacture enough supplies to meet what could be huge demand.
A vaccine was used to protect humans from a version of swine flu in the US in 1976.
However, it caused serious side effects, including an estimated 500 cases of Guillain-Barré syndrome. There were more deaths from the vaccine than the outbreak.
What should I do to stay safe?
Anyone with flu-like symptoms who might have been in contact with the swine virus – such as those living or travelling in the areas of Mexico that have been affected – should seek medical advice.
But patients are being asked not to go into GP surgeries in order to minimise the risk of spreading the disease to others. Instead, they should stay at home and call their healthcare provider for advice.
Countries around the world have taken varying measures but are mostly stepping up monitoring and issuing advice about travel to Mexico.
In the UK, the Foreign Office is advising against all but essential travel to Mexico.
What measures can I take to prevent infection?
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.
If caring for someone with a flu-like illness, a mask can be worn to cover the nose and mouth to reduce the risk of transmission. The UK is looking at increasing its stockpile of masks for healthcare workers for this reason.
In Mexico masks have been handed out to the general public, but experts are sceptical about how useful this is.
May it take some time for a pandemic to strike?
Possibly. The flu virus tends to thrive in cooler conditions, and to struggle in warmer weather.
The initial cases have developed right at the tail end of the winter flu season in the northern hemisphere, so it is possible that the number of infections may only begin to accelerate once the weather turns colder in the autumn.
However, the southern hemisphere is about to enter its winter season, and it is possible that the virus will take real hold there first.
Is it safe to eat pig meat?
Yes. There is no evidence that swine flu can be transmitted through eating meat from infected animals.
However, it is essential to cook meat properly. A temperature of 70C (158F) would be sure to kill the virus.
What about bird flu?
The strain of bird flu which has caused scores of human deaths in South East Asia in recent years is a different strain to that responsible for the current outbreak of swine flu.
The latest form of swine flu is a new type of the H1N1 strain, while bird, or avian flu, is H5N1.
Experts fear H5N1 hold the potential to trigger a pandemic because of its ability to mutate rapidly.
However, up until now it has remained very much a disease of birds.
Those humans who have been infected have, without exception, worked closely with birds, and cases of human-to-human transmission are extremely rare – there is no suggestion that H5N1 has gained the ability to pass easily from person to person.