When I was regular fasting and only re-feeding healthy (green leafies, good fats, limiting carbs, no junk, no sugar, only water to drink), I never got sick. This winter I derailed and got the flu (not Corona) for the first time I can remember in a long time. I wholeheartedly believe (why I derailed, I dunno), if done right, in the "you are what you eat" mantra.
Here's why fasting intrigued me, because I wanted to know how the body resorted to our fat stores as we modern day humans don't have to rely on that. Very often what we read as hunger is not hunger at all.