Old-growth forests are considered a benchmark for naturalness and models to which one compares managed forests, comparison leading to debates around biodiversity and its conservation, structure and dynamics, polarizing conservationists and forest practitioners. Plant pathogens are frequently disregarded as components of forests biodiversity, the common perception referring to this category of organisms as important biotic stressors. However, pathogens building several functional groups (necrotrophic and biotrophic pathogens, endophytic pathogens, sapro-parasitic species, generally on wood), in highly natural forests such as old-growth forests are establishing interaction networks with several other functional groups of organisms such as hyperparasites, consumers, disease facilitators, indirect opportunistic species (such as tree hollow dwellers), saprotrophs or mutualists. Being connected to old trees (rare or missing components of managed forests) or to endangered forest plants, pathogens become indicators of naturalness and biodiversity. The gradient going from saprotrophs, to sapro-pathogens and generalist/specialist pathogens characterizing forest ecosystems is linked at great extent to wood and bark, being connected to nutrient cycling as ecosystem level process. As long as disease is maintained within the baseline mortality of the trees, pathogens play the important role of control factors and contribute directly and indirectly to forest biodiversity. The current disease ecology progresses, the biodiversity integrative studies, and the new holistic approaches shaping modern ecology bring in the focus rare, endemic pathogens as important control factors of plant populations, as components of the plants’ phenotypic niche and of the global biodiversity, as potential providers of services (sources of medicines) and components of the intricate ecological webs. However, the comparisons between old-growth and managed forests biodiversity should be focused on species evenness (high evenness in old-growth forests versus low evenness in managed forests) and not particularly on species richness.  Still, there are pathogens responsible for major biotic disturbances in forests worldwide, the invasive, alien or emerging pathogens threatening both old-growth forests and managed forests.