If there have been a large number of such cases, it would mean the
virus is less
deadly than it might appear — but also more widespread in the community, and therefore
possibly more transmissible than we realise.
They report today in the Proceedings of the National Academy of Sciences that genes of the 1918
virus were most likely present in swine or human hosts at least 2 and
possibly 15 years before the pandemic began and combined to form the
deadly virus during multiple reassortments, presumably rare events in which flu
viruses exchange genes.