**6. A mechanistic link between human health and disease and the microbiome**

The microbiome can take up to 40% of our weight and can do many things. The human body is home to a microbiome, which is a networked community of microbes that outweigh the body's own cells. The human microbiome has piqued researchers' interest in recent years due to the microbiome's deep ties to human health. The human microbiome, also known as "our second genome," has developed alongside humans for millions of years and plays an important role in human health. Understanding the human microbiome's composition and function can help us better comprehend its structural and functional features. Understanding the human microbiome and applying metagenomic analysis to specific individuals will considerably improve our understanding of human health and diseases in the future. The study of the human microbiome and metagenome is seen as a new frontier in human genetics.

The majority of study on the human microbiome has focused on the microbes that colonize the human digestive system, as these microbes are thought to have a variety of effects on human health. The digestive system's microbiome is extraordinarily varied, with significant differences in its contents between individuals [50]. Extraneous variables, such as fecal transplantation and dietary intervention, have been proven to modulate the microbiome, which has been shown to be a viable therapeutic method to addressing a variety of health-related disorders [51]. The gastrointestinal tract (GIT) is home to a diverse range of microorganisms, which are connected by microbe-microbe and host-microbe interactions [52]. Microbial guilds (species that share resources) have been discovered to have intriguing traits that can help researchers better understand processes at both the single cell and community levels. Microbes are commensal and mediate digestion, enhance the immune system, and inhibit or prevent infections from penetrating the body under normal physiological conditions. The relationship between the human microbiome and human health is still largely unknown and unexplored, but a decrease in the diversity of the digestive system microbiota has been linked to diseases such as eczema [53], asthma, and inflammatory diseases [54], diabetes and obesity [55], allergies [56], and digestive tract disorders such as IBD (inflammatory bowel disease) [57], and IBS, according to a number of epidemiological studies (irritable

**Figure 3.** *Human microbial symbiosis has a close relationship with diseases of different systems [68].*

bowel syndrome) [58]. Chronic fatigue syndrome [59], cancer [60], colitis [61], and bacterial vaginosis [53, 54] have all been linked to dysbiosis (microbial imbalance). A number of recent studies have shown the importance of the gut microbiome in modifying immunological responses, including immune tolerance, via Treg (T regulatory) cell modulation. Short-chain fatty acids (SCFA) have been shown to increase the formation of Treg cells in the gut, according to Geuking et al. [62]. Microbes that live in the gut aid in the breakdown of complex carbohydrates and the usage of polysaccharides [5, 63]. Other health-promoting roles of the gut microbiome include immunological regulation [64], fecal microbiome transplantation [65], metabolism, xenobiotic toxicity, and pharmacokinetics, to name a few [66]. Therefore, patients with respiratory infections and diseases were shown to have gut dysbiosis and concomitant problems, showing gut-lung crosstalk, this phenomenon can also be seen in COVID-19 patients. As a result, boosting gut microbiota using probiotics and other beneficial bacteria is significant in therapeutic applications, and this could be extended to COVID-19 treatment as a new therapeutic approach according to Srinath et al [67] (**Figure 3**).
