Detailed discussion of infectious diseases, exploring various aspects of their causes, progression, and transmission. The first source focuses on defining and classifying diseases, highlighting the differences between signs and symptoms and providing a comprehensive overview of different disease types, including infectious, communicable, and non-communicable diseases. It also examines the stages of an infectious disease, emphasizing the incubation, prodromal, illness, decline, and convalescence periods. Finally, it explores the concepts of pathogenicity and virulence, explaining how these factors relate to disease development and severity. The second source dives into Koch’s postulates, a set of criteria used to establish a causal relationship between a pathogen and a disease. It examines the original postulates and their limitations, followed by a discussion of the molecular Koch’s postulates, a revised set of criteria that considers genetic factors. The source then delves into the stages of pathogenesis, including exposure, adhesion, invasion, and infection, highlighting the key mechanisms involved in each stage. It concludes by discussing the various modes of disease transmission and the importance of understanding how pathogens exit the body.