Mushrooms, long recognized for their nutritional and medicinal value, are emerging as a critical resource in the development of next-generation immunomodulators and antiviral agents. This chapter synthesizes recent advances in understanding the bioactive potential of fungal metabolites, particularly β-glucans, terpenoids, and glycoproteins, which exhibit dual immunoenhancing and direct antiviral properties. These compounds orchestrate immune responses by activating macrophages, modulating cytokine networks, and enhancing natural killer (NK) cell cytotoxicity, while concurrently disrupting viral replication through mechanisms such as envelope protein inhibition and viral protease suppression. Notably, their efficacy against RNA viruses, including influenza, SARS-CoV-2, and HIV, highlights their relevance in combating pandemic threats and drug-resistant pathogens. Breakthroughs in biotechnological and nanotechnological approaches are revolutionizing the application of these compounds. Advances in nanoencapsulation (such as exosome-based delivery systems) address bioavailability challenges, enabling targeted immune cell engagement and sustained release of hydrophobic metabolites like ergosterol derivatives. Meanwhile, CRISPR-based metabolic engineering and fungal heterologous expression systems are overcoming barriers to the sustainable production of rare bioactives. This convergence of disciplines is accelerating the translation of mushroom-derived molecules into functional foods, adjuvants, and standalone therapeutics. Despite progress, challenges persist in standardization, pharmacokinetic optimization, and clinical validation. Future research must prioritize mechanistic elucidation of host–pathogen–bioactive interactions, coupled with robust clinical trials to establish efficacy and safety. By integrating mycological biodiversity with modern biotechnological tools, this field holds promise for delivering scalable, eco-conscious solutions to global health crises, bridging traditional medicine with twenty-first-century innovation.