Coevolution between hosts and pathogens unfolds through reciprocal adaptations that select for virulence strategies and resistance traits, creating dynamic trajectories. This evergreen exploration examines ecological contexts, genetic mechanisms, and evolutionary feedbacks that guide how infections intensify, attenuate, or persist. By integrating theory with empirical patterns across systems, we reveal why virulence and resistance are not static traits but evolving outcomes influenced by population structure, transmission pathways, and ecological pressures. Understanding these processes helps anticipate outbreaks, manage resistance, and inform public health strategies in a changing world.