Not just what you eat, what you drink, breath & absorb through your skin.
Your body constantly eliminates waste & toxins that it absorbs. It can only process so much as organs typically perform multiple functions. Toxins that it can't process & excrete are largely stored in fat cells until you have the capacity to eliminate them.
I believe that often when we get 'sick' our bodies are simply overloaded with toxins. That is when we should be fasting & providing ways to help detox ourselves (eg activated carbon, bentonite clay, CDS).
We should avoid adding to our toxic load during such times (most pharmaceuticals are toxic).