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Influenza A viruses are a major public health concern due to their ability to swap genetic information and jump species, thus giving the viruses the ability to infect humans. This antigenic shift creates a new form of the virus that is different from the original pair that exchanged the genetic information. Historically, these mutated viruses cause the highly pathogenic and lethal pandemics. Variant type H5N1 is the subtype of most concern for public health officials due to its rapid rates of mutation and ability to acquire new genetic information.
Recent evidence suggests that the Spanish flu pandemic of 1918-19 that killed more than 500,000 people in the United States and more than 20 million worldwide was actually a highly pathogenic form of avian influenza (A-H1N1). The virus was recently re-created from samples collected from victims of the outbreak and currently resides in a CDC BL-3 containment facility.
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