[Petrochemicals (OP)]

Whilst reading about spanish flu, I read that the first wave of spanish flu was fairly standard, mailly dangerous to the old and inferm. I also read asia flu of the 1960s mutated into another form of the flu that wrought so much damage in 2017-18.

However the spanish flu virus mutated sometime in the following winter to become the world wide pandemic that is remembered. What factors led to this mutation ? Was it luck, is it simply what flu does or was it something to do with environmental factors (the trenches and WW1) ?

[evan_au]

What factors led to this ["Spanish" flu] mutation ?

At the time, the origin was shrouded in military secrecy due to WW1.

However, epidemiologists and geneticists have been able to look back into its origins, which appear to be in the USA.
- But it's politically charged, and thus rather contentious
- It does appear to be a reassortment mutation, when one individual animal is simultaneously infected by two different strains of influenza
- So it is is more likely to occur where many species are in close contact.
See: https://en.wikipedia.org/wiki/Spanish_flu#Hypotheses_about_the_source