Are swine flu or bird flu viruses dangerous
Most viruses that cause swine flu or bird flu are very hard to pass from one human to another: they don’t cause epidemics. Sometimes, however, further changes in genes create a virus that can spread rapidly among humans, and can produce a more severe illness. One reason this illness is more severe is that the virus is so new. The regular flu that comes each year is caused by a regular human influenza virus that often has similarities to the viruses that have caused the flu in years past, so people have some degree of immunity to the latest virus. The unusual swine flu or bird flu viruses that develop the ability for person-to-person spread are so different that people have little or no immunity to them. That is what some experts worry may be happening with swine flu.
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