Another thing I have read about the gut is that new research--at least to me--is showing that 70% of our immune system is in the gut.
So, it would stand to reason that, the healthier our gut, the healthier our immune system would be--which would then, make us less susceptible to colds and flu.
One thing that I have been doing for years--that I feel has helped me, is to take L-Glutamine powder.
I have read the it is good for leaking gut--and is a good immune system booster,
After trying many brands my favorite is Now Sports,
I prefer the powder, as it is easy for me to mix it into my protein shakes.
Hope this helps.