The microbiome (spectrum of bacteria in a patient's gut) is thought to play a role in helping to shape one's immune system given its direct contact with normal cells in our intestine. Recent studies have suggested that a microbiome with very few bacteria (low diversity) seems to be bad for health and a high-diversity microbiome (many different species) appears to be good. This appears to be true even for patients after HSCT where low diversity microbiome status has correlated with infections, GVHD and overall survival.