Epidemiology of Infectious Disease

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).