An healthy gut flora (of good bacteria) underpins the entire immune system
There is a constant war going on inside your small intestine (the stomach) between the good and bad bacteria for space on the walls of your intestine. If the good bacteria win, we will get nutrient absorption from our foods through the walls of the intestine - and into our bloodstream. If the bad bacteria (yeast or fungus) win, we will get not much nutrient absorption at all - thus chronic fatigue and other health issues will escalate.
The good bacteria thrive in an atmosphere of oxygen that come from mainly eating an abundance of raw vegetables and fruits - whereas the bad bacteria thrive in an atmosphere of carbon dioxide (mainly from eating too much cooked or processed foods) and sugar. So, which ever kind of foods we eat determines which kind of bacteria that will spread to cover the inside wall of the small intestine. You can literally have a yeast explosion inside of your intestines, because just as in the process of making beer or alcohol, yeast thrives when no oxygen and lots of sugar and carbon dioxide is present.
Also, consuming merely 30 teaspoons of refined sugar in one day is enough to wipe out our entire immune system for that day, because our immune system is too busy cleaning up that mess. One 350 ml can of soft drink, can contain as much as between 11-15 teaspoons of sugar. Add to that a few chocolate bars, cakes other sweets and whatever is inside our breakfast cereals, meals and coffee or teas and we can easily reach the detrimental level of 30 teaspoons every day of our lives for some of us.
Some have said that cancer is really "just a fungus!" that could not spread if we starved the cancer of sugar of any type. But sugar (especially sugars from carbohydrates) is everywhere in our society today. Another essential element that underpins the entire immune system is natural vitamin D3 from the sun's ultraviolet rays - but we seem to be overly paranoid about getting even a healthy dose of sunlight these days.