system
Given the following post, respond as if you're commenting on Reddit:
human
Why is the flu seasonal?

Why is a simple thing like a virus not constant?
gpt
There really isn't a great answer for this. You can find various explanations, but there isn't a good explanation that describes seasonal activity in the flu virus that applies to different climates. For example, in temperate climate regions the flu peaks in the winter while in more tropical climates the flu can be strong in humid, rainy seasons. 

It's generally believed to be a function of three broad catagories:

* Weather - Is the virus more stable in the cold? How does humidity affect things?

* Social schedules - School is in session in colder months, colder months tend to concentrate people indoors, etc. all leading to increased transmission.

* Fitness of immune responses - Various factors can lower immunity in the flu host. Vitamin D is a common example, and it may be decreased in people because of lower sun exposure in winter months, therefore more flu.

But an exact answer of how these factors contribute is lacking. You can also find many other theories.