"influenza viruses do not occur in all tropical regions of
the world."
"There are two other controversial theories for the appearance
of human influenza virus. These are controversial
because although they seem to explain the occurrence of
simultaneous disease outbreaks without discernable connection,
there is no biological evidence to support them. One
explanation, by Hope-Simpson and Golubev (65-67), is that
influenza viruses are spread by humans who are themselves
asymptomatic carriers, rather than directly from one acutely
infected host to the next."
"Alternative Theories for the Appearance of Human
Influenza Viruses
There are two other controversial theories for the appearance
of human influenza virus. These are controversial
because although they seem to explain the occurrence of
simultaneous disease outbreaks without discernable connection,
there is no biological evidence to support them. One
explanation, by Hope-Simpson and Golubev (65-67), is that
influenza viruses are spread by humans who are themselves
asymptomatic carriers, rather than directly from one acutely
infected host to the next. These carriers supposedly once
suffered an acute influenza infection. In these latently infected
carriers, seasonal changes correlated with solar irradiation
serve to reactivate the virus infection, but in rare
cases it may presumably remain dormant until many decades
have passed. "
"One noteworthy characteristic of the history of influenza
virus in humans is that when a new virus subtype appears
and causes a pandemic, the previously circulating subtype
usually disappears. This probably occurred in 1918 when
emerging HlNl viruses replaced H3-like viruses, in 1957
when the H2N2 subtype replaced HlNl, and 1968 when
H3N2 replaced H2N2. It did not occur in 1977, when HlNl
viruses reemerged after a 20-year absence from humans.
Since 1977, HlNl and H3N2 viruses have cocirculated.
The reasons for the sudden disappearance of previously
circulating human strains are unknown."
Webster et. al 2009
Evolution and Ecology of Influenza A Viruses
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC372859/pdf/microrev00028-0170.pdf