eli5: Why was the second wave of the Spanish Flu so much deadlier than the first?

Question

eli5: Why was the second wave of the Spanish Flu so much deadlier than the first?

In: Biology

Answer 1

[This video series explains the history of Spanish flu ](https://www.youtube.com/playlist?list=PLhyKYa0YJ_5BZ3gQleTk-PJqIejFf4Rh2) in a very ELI5 way, while being very complete and thorough.

Basically, we don’t know that it was deadlier for sure, though most evidence seems to point to that being true. There are quite a few hypotheses as to why that might be including viral mutations, troop movements, immunity from the first wave, etc.