Outbreaks can be categorized by the way that the causative agent (usually a micro organism) is spread within the population to reach new susceptible people. The place (or object) where people get infected by the micro organism is called 'the source' of the outbreak.

Broadly, outbreaks can be classified into two categories: common source and propagated. Common source outbreaks are further divided into point source, continuing source, and intermittent source outbreaks.

Common source outbreaks

When all (or most) cases in an outbreak are infected by the same source, then we call such an outbreak a common source outbreak.

Point Source Outbreaks

Common source outbreaks where the source has infected cases at one particular geographical location, during a short period of time, are called 'point source outbreaks'. In such situations the source is said to be located 'at a single point in time and place'. These outbreaks have a typical bell shaped epidemic curve, that increases sharply, peaks and then decline sharply, which reflects the normal distribution of the incubation period of the causative agent in humans. For this reason, the epidemic curve of a point source outbreak can help us identify the moment of transmission (i.e. when all cases have been exposed to the source).

Continuing common source outbreaks

When all (or most) cases in an outbreak have been infected by the same source, however over a prolonged period of time, then this type of outbreak is called 'continuing common source outbreak'. For example a recreational lake can be the source of leptospirosis to those who swim there. The shape of the epidemic curve of such outbreaks does not increase that sharply, it does not peak, yet reaches a plateau that is sustained over time, until the source is removed.

Intermittent Source Outbreaks

These outbreaks occur when exposure to the infectious agent happens sporadically, with periods of no exposure in between. An example would be a foodborne illness outbreak caused by a food product that is occasionally contaminated due to irregularities in the production process.

Propagated outbreaks

When an infectious disease is communicable (i.e. can be transmitted from person to person), then we can no longer consider that a single, common source is responsible for the outbreak. The causative agent is propagated within the population through human contact patterns. The shape of the epidemic curve in propagated outbreaks can vary and depends on the contact pattern and the proportion of susceptible individuals.

Classically, when a pathogen is introduced into a fully naive (i.e. fully susceptible) population with relatively easy person to person transmission (e.g. airborne), the epidemic curve increases sharply with incremental 'jumps'. The jumps reflect the generations of cases in the population.

FEM PAGE CONTRIBUTORS 2007

Editor

Arnold Bosman