Our digestive system is continuous with the atmosphere and accordingly exposed to bacteria, virus and what else is in the atmosphere. They get into the digestive system from the mouth to the anus and some reside, others just passing through. Particularly those that reside in the intestines are known as intestinal flora.
Intestinal flora may be good or bad bacteria, etc. Our digestive system needs good bacteria to help it break down food, digest and absorb the nutrients. Good intestinal flora is usually lactic bacteria and also known as probiotics - "good agents for life". The opposite would be anitbiotics - "against life" and kills all flora in the system, irrespective. One of the reasons you need to replenish after Antibiotics or a cleanse otherwise bad, harmful or pathogenic bacteria will flourish anc cause a myriad of problems.
Mother's milk contain lactic bacteria and it is known that babies fed mother's milk are often healthier and stronger and more resistant to illnesses. Likewise, adults too need good lactic bacteria, multi-strains to help keep maintain the digestive system. Equally important, help prevent harmful bacteria from colonizing and causing problems.