The document discusses the evolution of theories around disease causation from ancient supernatural and humorism theories to the germ theory of disease proposed by Louis Pasteur and Robert Koch in the 19th century. It then explains the limitations of the germ theory in explaining why some exposed individuals do not get infected, leading to the concept of the epidemiological triad of agent-host-environment interactions. The theory of multifactorial causation is introduced, recognizing that diseases often have multiple predisposing, exciting, and environmental factors. Modern chronic diseases especially involve complex webs of causal factors like lifestyle and behavior interacting over time.