Basic Infectious Disease Epidemiology

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Basic Infectious Disease Epidemiology
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What Causes an Infectious Disease?

• Infection is caused by a microorganism
• The microorganism may be a bacteria, a virus or a
  fungus

Bacillus anthracis bacteria Public Health Image
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Spread of infection

• Three conditions must be met for infection to
  spread from person to person
• If any of these conditions are not met, the chain
  is broken and the infectious disease does not
  spread.

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3 conditions required for infection to spread
from one person to another

• 1. One person must be infected with a
  microorganism
• 2. The other person must be susceptible to
  infection with that microorganism
• 3. The microorganism must be able to leave the
  body of the infected person and enter the body of
  the susceptible person.

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Susceptibility

• Ability to become infected with an organism when
  exposed to it
• People may loose susceptibility (or become
  immune) once they have been
• infected with the organism
• vaccinated
• Medications may reduce susceptibility temporarily

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Mode of transmission

• The way a microorganism is spread from an
  infected person to a susceptible person

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Modes of Transmission
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Direct transmission

• The spread of an organism from one person to
  another as a result of the direct contact of an
  infected and a susceptible person
• The modes of direct transmission
• direct contact
• droplet spread

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Direct contact

• The organism is passed to a susceptible person
  when he or she has contact with an infected
  persons skin, mucus membranes or body fluid
  (blood, urine, feces).

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Droplet spread

• The organism is spread to a susceptible person in
  respiratory secretions that form when an infected
  person breaths, coughs or sneezes.
• A susceptible person can become infected if the
  droplet enters his eyes, nose or mouth directly
  or via contamination of hands.

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Droplet Spread

• Considered direct transmission because droplets
  are heavy so they do not tend to travel more than
  6 feet from the infected person
• A susceptible person who is not near an infected
  person will not become infected
• The spread of disease by respiratory droplets
  usually requires extended contact

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Indirect transmission

• Occurs when an organism is spread from an
  infected person to a susceptible person via an
  intermediary or go-between.
• Via the air airborne
• Via an insect vectorborne
• Via an inanimate object vehicleborne

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Types of infection

• Colonization infection present on surface of
  body
• Organism propagating at a rate sufficient to
  maintain its numbers without producing
  identifiable evidence of any reaction in host
• Inapparent or subclinical infection
• organism not only multiplying but also causes a
  measurable reaction that is however not
  clinically detectable
• Symptomatic infection
• Organism causes clinically detectable reaction

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Time parameters of interaction

• Latency Period the time between infection agent
  and onset of infectiousness
• Incubation Period the time between infection
  and onset of symptoms
• Latency period may not be the same as the
  incubation period

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Time parameters of interaction
Webber,Communicable Disease Epidemiology and
Control, 1996. Figure 2.1