Epidemic

An epidemic is a marked increase in the number of cases of a disease relative to the expected number of cases. Epidemic disease is sometimes contrasted with endemic disease, which is the expected or usual incidence of disease in a location. While the term endemic is typically confined to infectious diseases, the term epidemic is more widely used. Endemic can refer to either the usually observed rate of disease or simply the fact that a disease is present in a locale. For example, hantavirus is endemic to many parts of the United States. A rate of disease that is endemic on one country would constitute an epidemic if it occurred in a country where the disease is ordinarily less common.

The terms outbreak and epidemic are both used to describe sudden increases in disease occurrence, although outbreak is usually reserved for a localized occurrence of a disease that is typically not present in the population (such as an outbreak illness due to Escherichia coli), while epidemic is reserved for more widespread conditions. Both outbreaks and epidemics may be described as being common source or propagated. In a common source outbreak, there is a single source of disease to which the population is exposed, such as cryptosporidium in a city water supply or the interstate distribution of contaminated packaged spinach. In a propagated epidemic, the pathogen is spread from person to person, such as HIV. Sometimes, an epidemic may have both types of transmission: initial point source exposure that is then passed from person to person.

A pandemic is an epidemic that has spread beyond national boundaries. Most often, the term pandemic is used to describe influenza, and the most notable pandemic in recent history was the Influenza Pandemic of 1918. However, other diseases such as cholera, bubonic plague, and HIV have similarly demonstrated rapid spread across continents at specific historical periods.

The progress of an epidemic or outbreak is often depicted with an epidemic curve, a graph where time is on the x-axis and the number of cases is on the y-axis. Figure 1 displays the epidemic curves of [Page 316]H5N1 avian (bird) flu in Thailand for two time periods in 2004.

Figure 1 Epidemic Curve of the Confirmed Highly Pathogenic Avian Influenza H5N1 Outbreaks in Poultry in Thailand by Date of Notification

Source: Tiensin et al. (2005).

Epidemics are most often caused by infectious agents such as viruses or bacteria, but other causes are possible, including chemical exposure and physical conditions such as extreme heat or cold. Due to improved sanitation, disease surveillance, and medical care, widespread epidemics of infectious diseases are [Page 317]uncommon in the industrialized world today but still occur regularly in the developing world. Due in part to this shift in focus from infectious to chronic disease, the term epidemic is often used today to refer to health behaviors or chronic conditions of widespread concern, such as ‘the obesity epidemic’ or ‘an epidemic of teenage smoking,’ or even ‘an epidemic of child abuse.’