The gut microbiota of most of us contains both “good bugs” and “bad bugs” which compete with one another. Populations of either increase at the expense of the other so when one is up, the other is down. Either the “good guys” or “bad guys” may predominate at different times and under different conditions.
“Bad” or “pathogenic” strains are associated with illness and disease. “Good bugs” are the opposite; they ensure health.
4 main things are understood to promote “dysbiosis” (i.e. when the “bad bugs” take over):
Sugar & refined carbohydrates in the diet
Low levels of fibre in the diet ("roughage" for us is food for the "good bugs")
Only 1 thing really helps promote microbiotic balance: Probiotics!