There's a difference. In the "hygiene hypothesis" model—the one where your immune system weakens without exposure—getting a cold makes you less likely to get the flu. That doesn't appear to be true. In reality, since the cold and the flu aren't biologically related, one does not confer immunity for the other.

That's not to say the "hygiene hypothesis" is completely false. There's good evidence that exposing children to low level pathogens early and often helps reduce autoimmune conditions like allergies, asthma, and arthritis. It just doesn't effect viral response.

I feel this is still framing it off? The hygiene hypothesis isn't just that your immune system weakens. It is also that you did not strengthen it. Not quite the same. And there is more to it than just getting targeted immunity to things. It could be that you get stronger nasal passages and lungs. Or whatever.

I'll underline that I don't know exactly what the answer is. I think there is plenty to research on this, and excited on us knowing a bit more.

The cold & flu were bad examples since they do share some similarities. A better example would be food poisoning and the flu: eating spoiled food doesn't make you less likely to catch a respiratory virus.

I think I agree with what you are saying. Paraphrased/simplified, the problem is that the extreme end of the hygiene hypothesis is to have a non clean area at all.

Reminds me of the somewhat silly phrase of "what doesn't kill you makes you stronger." Unless, of course, it leaves you much much weaker. Which is very possible.