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.