The human species and its ancestors have co-evolved with their microbiota/microbiome.
The existence of a core microbiome in all individuals indicates that this microbiota has conferred a selective advantage. Some of these microorganisms - pathobiotes - have pathogenic potential. Certainly... but the great equilibriums shaped by coevolution control their potential deleterious effects through counterbalancing by mutualistic symbiont microorganisms, confining them - among other things - to a role as inducers and regulators of immune responses (Th1, Th17 responses, etc.).
Hygiene: the loss of microorganisms, including pathogens, has created a situation favorable to the harmonious development of man. However, this loss of key microorganisms in ancestral balances seems to have created or facilitated the conditions for the onset of "postmodern" pathology, largely marked by a breakdown in inflammatory homeostasis: asthma/atopy, IBD, obesity... The "hygienist hypothesis" is thus emerging. Conversely, contemporary situations remind us of what the physiological consequences of poor hygiene must have been for "premodern" man. Such is the case with pediatric environmental enteropathy, the leading cause of malnutrition and stunted growth and psychomotor maturation in very young children in the most disadvantaged regions, such as sub-Saharan Africa, Madagascar and South Asia.
