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In addition to morbidity rates, the incidence and prevalence of mortality (death) may also be reported. A mortality rate can be expressed as the percentage of the population that has died from a disease or as the number of deaths per 100,000 persons (or other suitable standard number).

Graph of HIV prevalence and incidence in the US 1980-2010. The number o people living with HIV/AIDS was near 0 in 1980 and has increased steadily to over 1 million. There as a short plateau from 1990 to 1995. The number of new infections increased to nearly 200,000 in 1985 and dropped until 1990. It remains steady at somewhere near 50,000.
This graph compares the incidence of HIV (the number of new cases reported each year) with the prevalence (the total number of cases each year). Prevalence and incidence can also be expressed as a rate or proportion for a given population.
  • Explain the difference between incidence and prevalence.
  • Describe how morbidity and mortality rates are expressed.

Patterns of incidence

Diseases that are seen only occasionally, and usually without geographic concentration, are called sporadic disease s . Examples of sporadic diseases include tetanus , rabies , and plague . In the United States, Clostridium tetani , the bacterium that causes tetanus, is ubiquitous in the soil environment, but incidences of infection occur only rarely and in scattered locations because most individuals are vaccinated, clean wounds appropriately, or are only rarely in a situation that would cause infection. Centers for Disease Control and Prevention. “Tetanus Surveillance—United States, 2001–2008.” Morbidity and Mortality Weekly Report 60, no. 12 (2011): 365. Likewise in the United States there are a few scattered cases of plague each year, usually contracted from rodents in rural areas in the western states. Centers for Disease Control and Prevention. “Plague in the United States.” 2015. http://www.cdc.gov/plague/maps. Accessed June 1, 2016.

Diseases that are constantly present (often at a low level) in a population within a particular geographic region are called endemic disease s . For example, malaria is endemic to some regions of Brazil, but is not endemic to the United States.

Diseases for which a larger than expected number of cases occurs in a short time within a geographic region are called epidemic disease s . Influenza is a good example of a commonly epidemic disease. Incidence patterns of influenza tend to rise each winter in the northern hemisphere. These seasonal increases are expected, so it would not be accurate to say that influenza is epidemic every winter; however, some winters have an usually large number of seasonal influenza cases in particular regions, and such situations would qualify as epidemics ( [link] and [link] ).

An epidemic disease signals the breakdown of an equilibrium in disease frequency, often resulting from some change in environmental conditions or in the population. In the case of influenza, the disruption can be due to antigenic shift or drift (see Virulence Factors of Bacterial and Viral Pathogens ), which allows influenza virus strains to circumvent the acquired immunity of their human hosts.

An epidemic that occurs on a worldwide scale is called a pandemic disease . For example, HIV/AIDS is a pandemic disease and novel influenza virus strains often become pandemic.

A graph of the percentage of emergency department visits for influenza-like illness. The X axis is times of the year and the Y axis is percent. The national baseline is near 2.5%. All yeas have a small peak in January and a larger peak February to April. 2007-2008 had the largest peak in February to April. 2008 – 2009 had an additional peak from May to September.
The 2007–2008 influenza season in the United States saw much higher than normal numbers of visits to emergency departments for influenza-like symptoms as compared to the previous and the following years. (credit: modification of work by Centers for Disease Control and Prevention)

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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