The human microbiome exerts a central influence on cancer development, progression, and outcomes in critically ill patients. Dysbiosis, frequently triggered by chemotherapy, antibiotics, and intensive care interventions, disrupts epithelial barrier integrity and immune homeostasis, facilitating carcinogenesis and increasing susceptibility to sepsis. Mechanistic insights reveal how microbial communities modulate immune regulation, angiogenesis, and metabolic pathways, with specific taxa such as Fusobacterium nucleatum, Helicobacter pylori, and Escherichia coli implicated in oncogenesis. Epidemiological evidence suggests that nearly one-fifth of cancers worldwide are associated with microbial factors, while critically ill cancer patients face sepsis rates exceeding 30%, with high mortality linked to microbiome destabilization. Advances in diagnostic tools—including next-generation sequencing, microbial metabolomics, and tissue-specific profiling—are enabling precision approaches to identify at-risk patients and predict therapeutic responses. Beyond pathophysiology, the chapter underscores the translational relevance of microbiome-targeted strategies. Interventions such as antibiotic stewardship, fecal microbiota transplantation, short-chain fatty acid supplementation, fiber-enriched enteral nutrition, and cautious probiotic use have demonstrated potential to restore microbial balance and improve oncologic and infectious outcomes. Preservation of key commensals like Akkermansia muciniphila and Faecalibacterium prausnitzii is associated with enhanced immunotherapy efficacy, reduced graft-versus-host disease, and improved survival after hematopoietic stem cell transplantation. Ultimately, integrating microbiome-informed diagnostics and therapies into critical care and oncology offers a promising path toward precision medicine, positioning the microbiome not merely as a passive marker but as a modifiable determinant of prognosis in cancer patients.