Infectious diseases have posed significant threats to the public’s health for millennia. Despite a change in global and domestic leading causes of morbidity and mortality with a shift to chronic diseases, infectious diseases continue to impose a high burden on individual and population health.1 Therefore, public health practitioners must continue to attend to the prevention and control of infectious diseases. Common infectious diseases challenges in resource rich areas include influenza (which combined with pneumonia is the eighth leading cause of death in the United States), foodborne illnesses, healthcare-associated infections, sexually transmitted diseases, hepatitis C, and human immunodeficiency virus (HIV) infections.2 Although these same infectious diseases abound in lower resource countries, more traditional causes of infectious disease-associated morbidity and mortality prevail in these settings including tuberculosis (TB), malaria, soil-transmitted helminthic (STH) diseases, and diarrheal and acute respiratory infections (particularly in the under-5 age group). Adding to the complexity of the burden of infectious diseases is the identification of new infectious agents or the re-emergence of those in previously endemic areas. Demographic migrations among humans as well as animals, and the vectors and reservoirs of infectious agents, contribute to the introduction of new or the reintroduction of eliminated diseases as population threats where consideration of the classic epidemiological triad of determinants—the human host, the agent, and the human environment—comes into play.3
Selection and design of the optimal approach to prevent and control infectious diseases requires not only an understanding of the classic epidemiological triad but also the possible outcomes of the interactions of the host with a potentially infectious agent and the environment as it applies to both the host and the agent (Fig. 81-1). Although it is beyond the scope of this chapter to provide comprehensive information about every infectious agent, here we provide a general primer of infectious agents and their transmission to serve as a broad introduction to the subsequent communicable diseases chapters.
Chain of infection. (Source: Centers for Disease Control and Prevention. Principles of Epidemiology, 2nd ed. U.S. Department of Health and Human Services; 1992.)
Terminology and definitions associated with infection and infectious diseases often reflect perspective, including the public, the clinician, the public health practitioner, the clinical laboratorian, the research scientist, the historian, or the lawyer. Here, we will use definitions used in the scientific domains, including public health, with the understanding that this lexicon is not standardized. Infectious agents encompass both microscopic and macroscopic organisms. An agent that gains access to and causes a response in the host resulting in an end-result termed infection. Importantly, not all exposures of a host to an infectious agent result in infection. This may be due to the presence of an inhospitable setting wherein the agent is seeking entry to a nonpermissive anatomic site, the host may lack the needed receptor for the agent, ...