Download PH-100 Week 02 Review Epidemiologic Concepts Infectious Disease

Review of Epidemiologic Concepts in
Infectious Disease
Paul K. Mills, Ph.D., M.P.H.
The Infectious Disease Process
The Infectious Disease Process
Agents
Reservoirs
Portals of Entry and Exit
Transmission
Host Immunity
Herd Immunity
Herd immunity is the proportion of
resistant individuals in the population. If
the proportion is large enough, disease
transmission cannot occur.
Stages in the natural history of disease and
prevention
Spectrum
Stage
Prevention
Exposure Occurs
Stage of Susceptibility
Primary prevention
Incubation period
Stage of Subclinical Disease Secondary prevention
Pathologic changes occur
Onset of Symptoms
Time of Clinical Diagnosis
Stage of Clinical Disease
Stage of recovery, Disability
or Death
Tertiary prevention
Agent, Host, Environment
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Epidemics arise when the relative influence and balance of agent, host and
environment are thrown out of balance. Examples may include:
1. Introduction of a new agent.
2. Increases in the ability of the agent to survive in the environment.
3. Increases in the agents ability to infect the host (infectivity).
4. Increases in the ability of the agent to cause disease once inside the host
(pathogenicity).
5. Increases in the severity of disease once it has established itself inside the host
(virulence).
6. Increase in the proportion of susceptible in the population.
7. Environmental changes that favor growth.
8. Environmental changes that favor transmission of the agent.
9. Environmental changes that compromise host resistance.
Epidemiologic Concepts of Person, place and
time.
• Sporadic: occurring rarely and without regularity.
• Endemic: Occurring predictably and with only minor
or predictable fluctuations.
• Point epidemic: occurring in clear excess over time,
then rapidly returning to normal.
• Propagating epidemic: occurring in clear excess with
continuing increases over time.
• Pandemic: epidemics that affect several countries or
continents.