Since there are multiple types of microbes(near 40 trillion in the body) which microbe culture is promoted due to creating an environment that benefits specific type of microbe due to the gut content? When we look at that we see that gut environment effects immune homeostasis. There is constant back and forth suppressing and promoting (fighting-balancing act) due to byproducts created by the microbes. Further, gastrointestinal tract provides the largest surface area in the human body where microbes and their byproducts interacts with the immune system. When we look at breast cancer, we see increased specific type of bacterial transplantation occuring near the mammary glands. When we transplant gut contents that experiments show to promote cancer, we see double the rate occurring at experiments. Anyhow..if you are interested, read the 10 hallmarks.