How does stomach bug season start exactly?

[sunnyskies]

I get that once it does, it is so contagious that the way it passes from person to person is understandable. And with food poisoning, you know where that illness came from.

But how does the contagious stomach bug begin? The bacteria, the bug, whatever it is, where does it start with people? I don't know if that makes sense, haha. but I'm just really curious. Is there detailed reason for it or does it just kinda happen when people are in really close contact again after the summer, trapped inside, weather changes, etc.?

[JellyBn83]

I think that it is always around, but it can spread faster when a bunch of people are close together for a long period, as in schools. And when weather kicks in and people stay inside, that just adds to it.