Caused by influenza type A virus, 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.
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 thought to spread in the same way as seasonal flu – through coughing and sneezing.
The outbreak in Mexico seems to involve a new type of swine flu that contains DNA that is typically found in avian and human viruses.
The World Health Organization has confirmed at least some of the cases are caused by this new strain of H1N1.
"We are very, very concerned," World Health Organisation (WHO) spokesman Thomas Abraham said. "We have what appears to be a novel virus and it has spread from human to human... It's all hands on deck at the moment."
It is genetically different from the fully human H1N1 seasonal influenza virus that has been circulating globally for the past few years. It contains DNA that is typical to avian, swine and human viruses, including elements from European and Asian swine viruses
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.
The WHO is concerned but says it is too soon to change the threat level warning for a pandemic. However, it has convened an expert panel to consider whether to raise the alert level or issue travel advisories.
It might already be too late to contain the outbreak, a prominent US pandemic flu expert said.
Dr Michael Osterholm, a public health expert at the University of Minnesota, said given how quickly flu can spread around the globe, if these are the first signs of a pandemic, then there are probably cases incubating in other parts of the world already.
"Literally hundreds and thousands of travellers come in and out of Mexico City every day," Dr Osterholm said. "You'd have to believe there's been more unrecognised transmission that's occurred."
There is no vaccine that specifically protects against swine flu, and it was unclear how much protection current human flu vaccines might offer.
However, the CDC says two flu drugs, Tamiflu and Relenza, seem effective against the new strain. Roche, the maker of Tamiflu, said the company is prepared to immediately deploy a stockpile of the drug if requested.
Both drugs must be taken early, within a few days of the onset of symptoms, to be most effective. There have also been concerns that some forms of flu have been developing resistance to the drugs.
Scientists have long been concerned that a new flu virus could launch a worldwide pandemic of a killer disease.
A new pandemic flu virus could evolve when different flu viruses infect a pig, a person or a bird, mingling their genetic material. The resulting hybrid could spread quickly because people would have no natural defences against it.
The most notorious flu pandemic is thought to have killed at least 40 million people worldwide in 1918-19. Two other, less deadly flu pandemics struck in 1957 and 1968.
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