4

Many diseases of the respiratory tract present as having “Flu like symptoms” how do we know that the 1918 Spanish flu was a strain of influenza and not some virus with similar symptoms?

6

Because we know its genomic sequence.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2720273/:

Using fixed and frozen lung tissue of 1918 influenza victims, the complete genomic sequence of the 1918 influenza virus has been deduced. Sequence and phylogenetic analysis of the completed 1918 influenza virus genes shows them to be the most avian-like among the mammalian-adapted viruses. This finding supports the hypotheses that (1) the pandemic virus contains genes derived from avian-like influenza virus strains and that (2) the 1918 virus is the common ancestor of human and classical swine H1N1 influenza viruses

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