Virulence is thought to coevolve as a result of reciprocal selection between pathogens and their hosts. This paper focuses on coevolution between microbial proteinases operating as virulence factors and host defense molecules of insects. Owing to shorter generation times and smaller genomes, microbes exhibit a high evolutionary adaptability in comparison with their hosts. Indeed, the latter can only compete with pathogens if they evolve mechanisms providing a comparable genetic plasticity. Gene or domain duplication and shuffling by recombination is the driving force behind the countermeasures in host defense effectors. Recent literature provides evidence for both diversifications of fungal proteinases involved in pathogenesis and expansion host proteinase inhibitors subsets contributing to insect innate immunity. For example, the pathogen-associated spectrum of proteolytic enzymes encompasses thermolysin-like metalloproteinases that putatively promoted the evolution of corresponding host inhibitors of these virulence factors which complement the insect repertoire of antimicrobial defense molecules. Beyond mutual diversification of effector molecules coevolution resulted also in sophisticated molecular adaptations of host insects such as sensing and feedback-loop regulation of microbial metalloproteinases and corresponding countermeasures of pathogens providing evasion of host immunity induced by these virulence factors.