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).
- 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.