20.1 Principles of Epidemiology

1. Epidemiologists study the frequency and distribution of disease in order to identify its cause, source, and route of transmission.

Rate of Disease in a Population

1. Epidemiologists are concerned not simply with the number of people affected by a disease, but more importantly, with the rate of disease.

2. Diseases that are constantly present in a population are endemic; an unusually large number of cases in a population constitutes an epidemic. (Figure 20.2)

Reservoirs of Infectious Agents

1. Preventing susceptible people from coming in contact with a reservoir can prevent infectious disease.

2. Infected humans are the most significant reservoir of the majority of human diseases.

3. People who have asymptomatic infections or are colonized with a pathogen are carriers of the infectious agent.

4. Zoonotic diseases are those such as plague and rabies that can be transmitted to humans but exist primarily in other animals.

5. Pathogens that have environmental reservoirs are probably impossible to eliminate.

Portals of Exit

1. To spread, infectious microorganisms must exit one host to be transmitted to another.

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