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Swine flu cause:

Usually swine flu does not affect humans, but the current outbreak is apparently not solely due to swine influenza virus. It is due to a new Influenza A virus subtype H1N1, which passes with apparent ease from human to human.
It is important to realize that the influenza virus changes (mutates) constantly so that there are many strains of H1N1 that differ subtly from each other. Swine flu is caused by one strain of H1N1, but there are many other strains. Some H1N1 strains only infect pigs. Others infect humans, pigs, and birds. These subtle differences matter because the human body makes antibodies that are very tailored to a single strain of influenza. If a person recovers from swine flu, they are probably protected against infection from the same swine flu strain but are not protected against infections from other strains of H1N1 viruses.