Swine flu causes regular outbreaks in pigs
Infection control experts are scrambling to respond to outbreaks of swine flu in Mexico and the US, and suspected cases elsewhere.
What is swine flu?
Swine flu is a respiratory disease which infects pigs.
Caused by influenza type A, there are regular outbreaks among herds of pigs, where the disease causes high levels of illness but is rarely fatal.
It tends to spread in autumn and winter but can circulate all year round.
There are many different types of swine flu and like human flu, the infection is constantly changing.
Can humans catch swine flu?
Swine flu does not normally infect humans, although sporadic cases do occur - usually in people who have had close contact with pigs.
There have also been rare documented cases of humans passing the infection to other humans.
Human-to-human transmission of swine flu is thought to spread in the same way as seasonal flu - through coughing and sneezing.
In the latest outbreak it is clear that the disease is being passed from person to person.
Is this a new type of swine flu?
The World Health Organization has confirmed that at least some of the 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.
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.
Most cases so far reported around the world appear to be mild, but in Mexico lives have been lost.
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.
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 pandemic.
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 World Health Organization has warned that taken together the Mexican and US cases could potentially trigger a global pandemic, and stress that the situation is serious.
However, it is stressed that 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 - rating the threat at three on a six-point scale.
Nobody knows the full potential impact of the 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.
The fact that all the cases in the US have so far produced mild symptoms is encouraging. It suggests that the severity of the Mexican outbreak may be due to an unusual geographically-specific factor - possibly a second unrelated virus circulating in the community - which would be unlikely to come into play in the rest of the world.
Alternatively, people infected in Mexico may have sought treatment at much later stage than those in other countries.
It may also be the case that the form of the virus circulating in Mexico is subtly different to that elsewhere - although that will only be confirmed by laboratory analysis.
There is also 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 the virus be contained?
The virus appears already to have started to spread around the world, and most experts believe that containment of the virus in the era of readily available air travel will be extremely difficult.
What about treatments and vaccines?
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 around 33 million doses 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.
US scientists are already developing a bespoke new vaccine, but it may take some time to perfect it, and manufacture enough supplies to meet what could be huge demand.
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.