Human infections with avian influenza A viruses are uncommon but have occurred sporadically in many countries, usually after unprotected exposures (e.g. not using respiratory or eye protection) to infected poultry or virus-contaminated environments and have resulted in mild-to-severe illness with a wide range of symptoms and complications. A small number of human infections with avian influenza A viruses have been attributed to exposure to infected wild birds. For some human infections, the source of the virus infection was not determined. Human infections have occurred with different subtypes of low pathogenic and highly pathogenic avian influenza A viruses. The designation of “low” versus “highly” pathogenic avian influenza A virus refers to specific criteria, including mortality in experimentally infected poultry, and not to the severity of illness with human infections. Clinical illness associated with human infections with avian influenza A viruses does not necessarily correlate with virus pathogenicity in infected birds.
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