I teach classes about plant diseases to farmers and gardeners. One of the basic concepts I talk about is the disease triangle. Just as there are three sides to a triangle, there are three critical factors necessary for a disease to develop: a susceptible host, a pathogen and the environmental conditions to support the growth of the disease.
As an example, let’s pick on tar spot of corn and apple scab on crabapple. Both are fungal pathogens, and both diseases infect their susceptible hosts when the environmental conditions are humid or damp.