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Epidemiology is the study of the distribution and determinants of disease, both infectious and noninfectious, and other perturbations in health. Most epidemiologic studies of infectious diseases have focused on the factors that influence acquisition and spread with the goal of identifying methods for prevention and control. Epidemiologic studies have informed public health measures and thereby have been critical to the control of epidemics, such as those due to cholera, plague, smallpox, yellow fever, and typhus. Knowledge of the principles and practice of epidemiology is essential for clinicians (those treating individual patients) and public health practitioners (those focused on the health of the community) alike. Care of patients with suspected infections requires consideration of the likelihood of possible exposures in the community (acquisition) and to the community (spread to others). For example, what infections, especially viral, are currently circulating in the community? Has the patient traveled recently to an area where other infections are present? Is a nosocomial or other healthcare-associated infection possible because the patient has been hospitalized recently or resides in a long-term care facility? Does the patient’s infection pose a risk to his/her family, school- or workmates, or friends?


An emerging disease is an infectious disease whose incidence has increased in the past two decades and/or that threatens to increase soon. Emerging infectious diseases reflect the arrival of a new pathogen (newly emerging) or an old pathogen that is increasing in incidence, clinical or laboratory characteristics, or geographic range (re-emerging or resurging). An unusual third group is “deliberately emerging” infections, such as anthrax bioterrorism. The appearance of novel coronaviruses (eg, the severe acute respiratory syndrome [SARS] coronavirus and now SARS-CoV-2 [the cause of COVID-19]) are examples of new pathogens, multidrug-resistant Mycobacterium tuberculosis represents an old pathogen with new characteristics, and cholera and Zika in the Americas are examples of old pathogens with a new geographic range (Asia to South America). New methods of detection (eg, molecular) and surveillance (eg, global) have greatly improved our ability to detect and characterize emerging and reemerging infectious diseases. The fundamental methodologies of molecular epidemiology are described in Chapter 4, and their specific applications are discussed in many other chapters throughout this book.

Some factors that increase emergence or reemergence of infectious pathogens include:

  • Human and animal demographics and population movement with intrusion into new habitats (particularly tropical forests)

  • Irrigation, especially primitive irrigation systems, which fail to control arthropods and enteric organisms

  • Uncontrolled urbanization, with vector populations breeding in stagnant water

  • Increased international commerce and travel with contact or transport of vectors and pathogens (globalization)

  • Breakdown in public health measures, including sanitation, vector control, immunization programs related to social unrest, civil wars, and major natural disasters

  • Ecological changes, including global climate change and deforestation, with farmers and their animals exposed to new arthropods, floods, and drought

  • Microbial evolution whether related to indiscriminate use of anti-infective ...

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