Spread of a microbial disease through a population of individuals can be considered as vertical (transferred from one generation to another) or horizontal (transfer occurring within genetically unrelated groups).
EPIDEMIOLOGY OF INFECTIOUS DISEASE
Spread of a microbial
disease through a population of individuals can be considered as vertical (transferred from one
generation to another) or horizontal
(transfer occurring within genetically unrelated groups). The latter can be
divided into common source outbreaks,
relating to infection of a number of susceptible individuals from a single
reservoir of the infective agent (i.e. infected foods), or propagated source outbreaks, where each individual provides a new
source for the infection of others.
Common source outbreaks
are characterized by a sharp onset of reported cases over the course of a
single incubation period (Figure 7.2) and relate to a common experience of the
infected individuals (e.g. a contaminated food product). The number of cases
will persist until the source of the infection is removed. If the source
remains (i.e. a reservoir of insect vectors) then the disease becomes endemic
to the exposed population, with a constant rate of infection. Propagated source
outbreaks, on the other hand, are brought about by person to person spread, and
show a gradual increase in reported cases over a number of incubation periods
and eventually decline when most of the susceptible individuals in the
population have been affected (Figure 7.2). Factors that contribute to
propagated outbreaks of infectious disease are the infectivity of the agent
(I), the population density and the numbers of susceptible individuals in it
(F). The likelihood of an epidemic occurring is given by the product of these
three factors (i.e. FIP). An increase in any one of them might initiate an
outbreak of the disease in epidemic proportions. Thus, reported cases of
particular diseases show periodicity, with outbreaks of epidemic proportion
occurring only when FIP exceeds certain critical threshold values, related to
the infectivity of the agent. Outbreaks of measles and chickenpox therefore
tend to occur annually in the late summer among children attending school for
the first time. This has the effect of concentrating all susceptible individuals
in one, often confined, space at the same time. The proportion of susceptible
individuals can be reduced through rigorous vaccination programmes. Provided
that the susceptible population does not exceed the threshold FIP value, then
herd immunity against epidemic spread of the disease will be maintained.
Certain types of
infectious agent (e.g. influenza virus) are able to combat herd immunity such
as this through undergoing major antigenic changes. These render the majority
of the population susceptible, and their occurrence is often accompanied by
spread of the disease across the entire globe (pandemics).
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