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Chapter 15: Epidemiology and Infectious Diseases
•Study of factors and mechanisms involved in the frequency and spread of diseases and health-related
•Includes relationships among pathogens, their hosts, and the environment
The assignment or study of causes and origins of a disease
Incidence and Prevalence Rates
•Incidence of a disease: the number of new cases contracted within a set population during a specific
period of time
•Prevalence of a disease:
the total number of people infected within the population at any time
Diseases in Populations
•Endemic: Present continually in the population of a particular geographic area
•Epidemic: Arises when a disease suddenly has a higher-than-normal incidence in a population
•Pandemic: Occurs when an epidemic spreads worldwide
•Sporadic: Occurs in a random and unpredictable manner
Reservoirs of Infection
•Sites in which organisms can persist and maintain their ability to infect
•Are essential for new human infections to occur
•Other animals (including insects)
•Certain nonliving material (water and soil)
Human Reservoirs
•Carriers: individuals who harbor an infectious agent without having any observable clinical signs or
•Chronic carrier: is a reservoir of infection for a long time after he or she has recovered from a disese
•Intermittent carrier: periodically releases infectious organisms
Animal Reservoirs
•About 150 pathogens can infect both humans and some other animals
diseases that can be transmitted under natural conditions to humans from other vertebrate
•Table 15.1
Modes of Disease Transmission
•Contact Transmission
•Vehicle Transmission
•Vector-Borne Transmission
Contact Transmission
•Can be direct, indirect, or by droplets
•Direct contact requires body contact between individuals (horizontal or vertical)
•Indirect contact occurs through fomites
•Droplet: occurs when a person coughs, sneezes, or speaks near others
Transmission by Vehicles
•Nonliving carrier of an infectious agent from its reservoir to a susceptible host
Transmission by Vectors
•Vectors: Living organisms that transmit disease to humans
•Most vectors are arthropods:
ticks, flies, fleas, lice and mosquitoes
Special Problems in Disease Transmission
•Carriers usually do not know they are carriers and are often difficult to identify
•Sexually transmitted diseases present epidemiological problems because infected individuals may have
multiple partners
•Zoonoses are another epidemiological problem
Controlling Disease Transmission
•Isolation: A patient with a communicable disease is prevented from having contact with the general
•Quarantine: the separation of “healthy” human or animal carriers from the general population when
they have been exposed to a communicable disease
Public Health Organizations
•The Centers for Disease Control and Prevention (CDC)
•The World Health Organization (WHO)
Nosocomial Infections
•An infection acquired in a hospital or other medical facility
•Exogenous infections:
•Endogenous infections:
caused by organisms that enter the patient from the environment
caused by opportunists among the patient’s own normal microbiota