Fish, as the earliest vertebrates possessing both innate and adaptive immune systems, provide a unique window into the evolutionary and ecological modulation of vertebrate immunity within pathogen-rich aquatic environments. Innate defences (skin, gills, mucus), humoral factors (lysozyme, complement, antimicrobial peptides), and cellular effectors (macrophages, neutrophils) deliver rapid protection, while adaptive immunity supplies specificity and memory through immunoglobulins (IgM, IgD, IgT/IgZ), T cells, and mucosal-associated lymphoid tissues (MALT). Environmental stressors (temperature extremes, salinity shifts, hypoxia, pollutants and climate change) disrupt this integrated system, often elevating cortisol, inducing oxidative stress and microbiome dysbiosis, and thereby increasing disease susceptibility. To move beyond descriptive catalogues of these effects, this review organises existing knowledge within an explicit geo-eco-immunology framework structured along three axes: habitat type (freshwater versus marine), latitudinal/climatic gradient (tropical versus polar), and anthropogenic pressure (natural populations versus intensive aquaculture and polluted systems). Along these axes two broad, non-exclusive immune-investment strategies emerge: mucosal-dominant, plastic responses favoured in pathogen-rich, osmotically stable marine and tropical settings, and more systemic, metabolically constrained responses characteristic of cold, high-altitude or chronically stressed environments. Comparative synthesis indicates that freshwater species generally emphasise robust systemic innate defences against opportunistic pathogens such as Aeromonas hydrophila, whereas marine teleosts exhibit specialised mucosal adaptations against Vibrio and related taxa. Tropical fishes tend toward broader, more plastic repertoires under high pathogen diversity; polar and high-altitude species prioritise metabolic efficiency. Aquaculture intensification and pollution amplify these natural stressors, producing consistent patterns of immune suppression across multiple independent studies, although inter-species variation and interactive effects remain incompletely quantified. By critically evaluating methodological heterogeneity and evidence strength, the review distinguishes robust tendencies from context-dependent or weakly supported claims. It highlights tools (omics, biomarkers, GIS) and applications for region-specific vaccines, immunostimulants, broodstock selection, conservation prioritisation and fisheries management, while identifying critical gaps in geographic coverage, multi-species comparative data and long-term field validation. Strengthening the geo-eco-immunology framework will improve prediction of disease dynamics under climate change and support more sustainable aquaculture and aquatic biodiversity resilience.