The speculative scenario popularized in modern culture—wherein an entomopathogenic fungus such as Ophiocordyceps unilateralis adapts to parasitize humans and commandeer mammalian motor behavior—presents a profound biophysical paradox. While conventional science dismisses macroscopic human "zombie" parasitism as biological fantasy, the underlying molecular reality is far more nuanced. Fungi already exert potent, evolutionary-conserved control over human neurochemistry, ranging from the ergot alkaloid-induced psychomotor hysterias of Claviceps purpurea and the 5-HT2A serotonin receptor agonism of psilocin to the subtle neuro-endocrine modulation of the human gut-brain axis by Candida species. In this paper, we conduct a rigorous cross-disciplinary examination of the physiological, thermodynamic, and immunological barriers that currently prevent entomopathogenic fungi from colonizing the human central nervous system. Specifically, we dissect the Mammalian Triad of Protection: (1) the endothermic thermal barrier (37°C basal core temperature), (2) the structural and cellular vigilance of the Blood-Brain Barrier, and (3) the extreme neuromuscular complexity of mammalian motor endplates compared to arthropod hemolymph systems. We then evaluate whether climate-driven thermal adaptation—as empirically demonstrated by the spontaneous emergence of thermotolerant Candida auris—could erode this thermal moat under continued planetary warming. Finally, we explore the biosecurity frontier: the consequences of potential barrier breaching via synthetic mycology and the critical governance deficits surrounding mammalian mycoses. We conclude that while natural evolution is unlikely to yield rapid mammalian behavioral hijacking, the climate-driven erosion of our thermal moat and refractory neurotropic mycoses represent an overlooked biological frontier requiring urgent international biosafety surveillance.