Chapter 14: Principles of Disease and Epidemiology

Principles of Disease and Epidemiology begins by differentiating between infection (colonization by pathogens) and disease (a change in health resulting from infection). Key terms are clarified, including pathogenesis (the development of disease), etiology (cause), and the distinctions between communicable, contagious, and noncommunicable diseases. The chapter outlines how normal microbiota—resident and transient—play a protective role and contribute to microbial antagonism and symbiosis (commensalism, mutualism, parasitism). The concept of opportunistic pathogens is introduced alongside conditions that can disrupt microbial balance and allow disease to occur, such as immunosuppression and changes in host environment. The chapter discusses Koch’s postulates, which provide a framework for linking specific microbes to specific diseases, and updates them with modern molecular Koch’s postulates for microbes that can’t be cultured. Next, the chapter classifies disease occurrence (sporadic, endemic, epidemic, pandemic), severity (acute, chronic, latent), and scope (local, systemic, focal, sepsis). It details the stages of disease progression: incubation, prodromal, period of illness, period of decline, and convalescence. Reservoirs of infection (human, animal, and nonliving) and modes of disease transmission are examined, including direct contact, indirect contact via fomites, droplet, vehicle transmission (e.g., waterborne, foodborne), vectors (mechanical and biological), and nosocomial (healthcare-associated) infections. Epidemiological tools and terminology are covered, including incidence, prevalence, morbidity, and mortality rates. The roles of the CDC, WHO, and local agencies in disease tracking and outbreak response are highlighted. The chapter introduces descriptive, analytical, and experimental epidemiological methods used to identify causes and transmission patterns of disease. Case reporting, notifiable diseases, and emerging infectious diseases are emphasized, particularly those influenced by genetic recombination, climate change, travel, and antimicrobial resistance. This chapter equips students with a comprehensive understanding of how diseases originate, spread, and are tracked—forming the foundation of infection control, clinical microbiology, and public health surveillance.