Introductory Chapter: Human and Microbes in Health and Diseases

Microbes are ubiquitous in nature and humans are no exception. Microbes have coevolved with humans and reside in and on human body to develop a host associated structure, called “Human Microbiome” or “Human Microbiota.” These microbial counterparts account toward 10% of human body weight and outnumber human cells by approximately by tenfold and considered as commensals. Human microbiome is defined as the total genomes of microbes (constitute bacteria, bacteriophage, fungi, protozoa and viruses) that live inside or on the human body [1]. There are trillions of microbes living in/on human body plays a fundamental role in normal functioning of metabolic, physiological and immune system. Microbiota is a complex ecosystem consisting of bacteria, protozoa, viruses and fungi; all varies in number even in body parts of same individual. Human body has 10 times more bacteria than the number of human cells in our body [2]. Most of these bacteria are present in gastrointestinal tract [3] which account for approximate 70% of the total microbial load in or on human body (particular in large intestine) [4]. Humans are born sterile and start acquiring human companion to shape resilient microbiome structure. Establishment of microbiome starts with birth and matures with age. Microbial introduction and the establishment of microbiome is a random process influenced by many factor like mode of delivery, diet, sex, age, genetics, geographical location have a strong impact in shaping human microbiome structure [5–10]. These microbes are in symbiotic relationship, beside gut they are also found in mouth, respiratory tract, vagina and skin.