Out of everything we talked about (food, nutrition, etc.), stress has the most profound effect on your immune system.
If you are nourishing your body, and you are on the best kind of diets, but you still get sick, that is the indication that there is stress stored in your mind. If you are unaware of it, or you can't find the reason you are stressed out, dig into the subconscious mind and your history to find it. The subconscious mind CANNOT forget. Every stressful trauma you experienced ten, twenty, or fifty years ago is still living there, and it will remain alive unless you dissolve it.
So, dealing with stress might be the solution you've been looking for. It sure is a good thing to do, because people that are under stress are like magnets for viruses and bacterial infections. It (stress) is the number one thing that triggers autoimmune diseases.
What Does Stress Do To Your Body?
Cortisol spikes shut down white blood cells. That is called LYMPHOCYTOPENIA (lymphocytes are the white blood cells). Cortisol spikes thanks to stress, so, it is the right thing to say that stress shuts down T-cells, B-cells, antibodies, phagocytes, NK cells, or simplified: stress shuts down the entire immune system.
Stress also shrinks the size of lymph nodes (where the immune reaction occurs) and the thymus gland.
There are different kinds of stress you might go through (fear, worry, etc.), but the most damaging kind is called bereavement. That would be the loss of loved ones, a divorce, a loss of a job, etc. This shits down your NK cells and increases your susceptibility to infections (and the duration of sickness).
If you are under stress, the first thing you should do is to isolate the cause (stress is transmissive, or in other words, you may be affected by other people's stress).
What helps are long walks, frequent exercises, more sleep, and a peaceful/joyful environment. Try not to listen to music that contains negative affirmations (lyrics).
Vitamin B1 may help as well. it is the most important nutrient to reduce stress!