It was caused by a particular strain of the influenza virus, just like any other flu. This particular strain A-H1N1/09 (aka: Influenza A, Novel H1N1 or 2009 Swine Flu virus) originated in pigs (swine) but has changed itself to be infective to humans.
Swine flu is a virus that originally infected only pigs and they spread it from one infected hog to other hogs the same way it spreads in people, by direct contact or by droplets holding the virus in the air after a pig coughed or sneezed. Because pigs are physiologically very similar to humans, when they have been living closely with humans, some microbes that can infect them are able to be changed or mutated into strains that people can catch. These types of diseases that we can get from animals are called zoonotic diseases.
Pigs contract swine flu from other infected swine, particularly in pigpens and herding areas. Flu viruses from birds and people may also infect pigs. Swine influenza passes quickly among pigs, and can cause major losses for pig farmers because of the rate of illness. It is now also possible for infected people to pass swine flu back to other pigs.
Usually viruses that infect one kind of animal do not often infect others. However, if the animals are kept very closely together, it is more likely that mutations can occur that allow new strains to develop with the ability to cross from one type of animal to another. Viruses can mutate very rapidly. Since they are non-living sub-microscopic organisms (microbes), and not actually living organisms (like bacteria and microscopic fungi), they can combine with the cells of host animals and change the genetic material in those host cells to reproduce themselves. Sometimes that assimilation can also change the makeup to a new strain of virus (sub-microscopic particle microbe) which is then replicated by the damaged cells as well.
This cross-family type of mutation is called a reassortant, or sometimes reassortment, of genetic material in viruses. It has happened in the past when ducks or other birds were kept very closely together with pigs. That is how the Avian (bird) flu became a problem for people. First bird flu was mutated to a strain that could be caught by the pigs they were closely around. Once infecting the pigs, other mutations occurred that allowed their bird flu-swine crossed viruses to mutate again to viruses that people who were in close contact with the pigs infected with the bird-swine virus could catch. This kind of "cross-contamination", through mutations of the viruses infecting the physiologically similar pigs, allows new strains to develop to which people have no inherited immunity (passed down from generation to generation) or prior exposure to a similar type that would have given them cross protection.
It is considered potentially more dangerous than other types of influenza because the human population has not experienced this particular kind of Swine flu before. Therefore, it is anticipated that there will be few people with any natural resistance to it, whereas most people usually have some resistance to other strains of influenza once they are 10 years old or older. As a result of the lack of resistance, it can spread more easily and perhaps produce more severe symptoms.
The 2009 Swine Flu was doubly difficult for us to create effective vaccines (which would help to teach our immune systems how to fight the virus). This is because the new strain of virus mutated within the pigs where it could first merge genetic materials of the bird viruses and swine viruses that the pigs had been exposed to and then it become infective to humans as well, from the additional close contact of pigs and humans who cared for them (triple reassortant). The reassortant process within the pigs combined the pig genetic material, the bird genetic material, and also human genetic material. The 2009 Pandemic Swine Flu virus (A-H1N1/09) contains genetic material that is from the bird flu as well as from three swine flu virus strains (Asian, American, and European), plus the human flu virus ("quintuple reassortant"). We would have had an easier time developing our vaccine for this flu virus if it had been formed from one or the other, and not with the five types of genetic material that it currently contains. As it is, we had to start "from scratch" to grow the right kind of virus to put into the vaccines.
The whole process of infections can also work back the other way. So now we humans can get swine flu from each other, from infected pigs, and we can give it to pigs who can infect each other. In other words, because aspects of human viral strains are incorporated into the animal viral strains using the pig as a "middle man", newer viruses are developed that are able to cross the animal family boundaries.
Over crowding, of people with each other and with pigs, and pigs with each other and with other animals, are the major reasons these "reassortant" viruses can occur.
Farmers who work with swine must use very clean techniques to avoid catching and transmitting the swine flu. The CDC has guidelines for people who have these jobs or spend time around hogs, such as showing them in breed competitions. See the links below.
As with any virus, very good hygiene, including thorough hand washing, is critical after contact or close proximity.
See the related questions for steps to take to avoid contracting this virus.
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