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Epidemiology, the study of the distribution of determinants of disease and injury in human populations, is a discipline that includes both infectious and noninfectious diseases. Most epidemiologic studies of infectious diseases have concentrated on the factors that influence acquisition and spread, because this knowledge is essential for developing methods of prevention and control. Historically, epidemiologic studies and the application of the knowledge gained from them have been central to the control of the great epidemic diseases, such as cholera, plague, smallpox, yellow fever, and typhus.
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An understanding of the principles of epidemiology and the spread of disease is essential to all medical personnel, whether their work is with the individual patient or with the community. Most infections must be evaluated in their epidemiologic setting. For example, what infections, especially viral, are currently prevalent in the community? Has the patient recently traveled to an area of special disease prevalence? Is there a possibility of nosocomial infection from recent hospitalization? What is the risk to the patient's family, schoolmates, and work or social contacts?
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The recent recognition of emerging infectious diseases has heightened appreciation of the importance of epidemiologic information. A few examples of these newly identified infections are cryptosporidiosis, hantavirus pulmonary syndrome, and severe acute respiratory syndrome (SARS) coronavirus disease. In addition, some well-known pathogens have assumed new epidemiologic importance by virtue of acquired antimicrobial resistance (eg, penicillin-resistant pneumococci, vancomycin-resistant enterococci, carbapenem-resistant enterobacteraciae, and multiresistant Mycobacterium tuberculosis).
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Over the past two decades, powerful new molecular methods have been developed that have greatly enhanced the ability to even more clearly understand the origins, evolution and spread of a wide variety of infectious agents. This discipline is called molecular epidemiology. The fundamental methodologies are described in Chapter 4, and their specific applications are discussed in many other chapters throughout this book.
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Factors that increase the emergence or reemergence of various pathogens include:
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Population movements and the intrusion of humans and domestic animals into new habitats, particularly tropical forests
Deforestation, with the development of new farmlands and exposure of farmers and domestic animals to new arthropods and primary pathogens
Irrigation, especially primitive irrigation systems, which fail to control arthropods and enteric organisms
Uncontrolled urbanization, with vector populations breeding in stagnant water
Increased long-distance air travel, with contact or transport of arthropod vectors and primary pathogens
Social unrest, civil wars, and major natural disasters, leading to famine and disruption of sanitation systems, immunization programs, etc.
Global climate change
Microbial evolution, leading to natural selection of multiresistant agents (eg, methicillin-resistant staphylococci; new, highly virulent strains of influenza A virus). In some instances, these changes can be accelerated considerably by indiscriminate use of antiinfective agents.
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There are other factors, of course, and all of these are discussed here as to their relative impacts on the specific infectious agents described in subsequent chapters.
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