Influenza virus: There are many strains (family members) of influenza virus all over the world. Some strains affect animals and others humans. When a strain that "normally" infects animals develops the ability to infect humans, that is potentially dangerous because humans may have never had to fight off (become immune to) that strain. The "avian flu strains" come from birds. Swine flu strains came from pigs.
Answered 6/11/2017
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