I get asked this question sometimes.
I’m not a doctor.
However, barring any severe, rare, or specific medical conditions that need professional treatment from a qualified practitioner, here’s what I can add:
If you’re generally an ‘average’ person without those kinds of medical conditions, then you should look at your nutrition, movement practice (or lack of), and stress levels.
These things, if left to be ‘just average’ or worse, will make you sick.
Matt Fried 