system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 - How can some flu/cold seasons be worse because previous years were better?
gpt
All flu viruses are related to each other, which means that, to our body, they may also appear similar to each other. If you got Flu-1 last year (and recovered) and Flu-2 is coming around this year is a variant of Flu-1, you may have some residual immunity from Flu-1 that can help you fight Flu-2.

However, if very few people got the flu last year and the year before that, then their previous immunity (from three winters ago) is much reduced. Also, since the flu has been able to evolve for two years, it may look more different to our bodies than last-year's flu might look. This makes the current form of the flu more challenging for our bodies to handle.