This book highlights the hidden world of microbiomes, viromes, and biofilms on and inside humans. Our bodies have microbiomes, which contain various bacterial communities, and viromes, which contain viruses. These invisible residents are vital to health and sickness. Infections often result from microbiome imbalances. Pathogenic bacteria can outcompete beneficial ones, causing gut dysbiosis and disease. Understanding this interaction allows probiotics to restore balance and fight infections. Microbes have a major impact on the gut-brain axis. Gut bacteria affect mood, behavior, and neurodegenerative illnesses by producing neurotransmitters and influencing brain development. The "diabesity" relationship to microbiome changes underscores its relevance in metabolic illnesses like diabetes and NAFLD. The microbiota can also affect cancer progression. Specific bacterial strains may promote tumor growth or prevent metastasis, enabling microbiome-based cancer treatments. Probiotics—live bacteria—have enormous potential. Beneficial strains can improve intestinal balance, fight infections, and treat diabetes and autoimmune conditions. Fecal transplants or co-sleeping may modify microbial ecosystems and health effects. Microbiomes, viromes, and biofilms are complex, requiring advanced tools. The makeup, function, and interactions of these communities can be characterized using next-generation sequencing (NGS). As NGS technology advances, researchers explore this hidden world, offering a future where microbiome manipulation underpins customized therapy and preventive healthcare. This area has great potential for understanding health, improving well-being, and fighting diseases in innovative ways.