Some
analysis has been done on the sequence of
1918 influenza strains. It is thought that it was a H1N1
strain of influenza A, known also as "
swine flu". It is thought that the
virus was particularly
virulent due to a mutant strain of the
hemagglutinin (HA) surface
protein it carried. For activation, HA must be cleaved by a host
protease. The normal protease responsible is found in the
respiratory tract of humans, or the
digestive tract of
avians. Some mutants of
avian flu contain different sequences at the
cleavage point, allowing them to be cut by a wide variety of proteases, allowing the virus to be
activated anywhere in the
body. It is not known for certain that this is responsible for the virulence of the 1918 strain.
The most popular speculation for why the "
Spanish Flu" seemed to
kill the most healthy individuals is that the
immune response itself was responsible for the
pneumonia that killed many. A stronger immune response, like that of an
adult in their 20s, would result in more rapid
secretion of
mucus in the
lungs, causing the victim to essentially
drown in it.