11:32 am - Monday May 28, 2012


epidemicIn epidemiology, an epidemic occurs when new cases of a certain disease, in a given human population, and during a given period, substantially exceed what is expected based on recent experience. Epidemiologists often consider the term outbreak to be synonymous to epidemic, but the general public typically perceives outbreaks to be more local and less serious than epidemics.

An epidemic may be restricted to one locale, however if it spreads to other countries or continents and affects a substantial number of people, it may be termed a pandemic. The declaration of an epidemic usually requires a good understanding of a baseline rate of incidence; epidemics for certain diseases, such as influenza, are defined as reaching some defined increase in incidence above this baseline. A few cases of a very rare disease may be classified as an epidemic, while many cases of a common disease (such as the common cold) would not.

Causes of Epidemics

There are several changes that may occur in an infectious agent that may trigger an epidemic these include:

  • Increased virulence
  • Introduction into a novel setting
  • Changes in host susceptibility to the infectious agent
  • Changes in host exposure to the infectious agent

An epidemic disease is not required to be contagious, and the term has been applied to West Nile fever and the obesity epidemic, among others.

Effects of Epidemics

They can cause people to die/get sick, loss of population and tourisim, businesses can go down hill (cause ppl will have to stay home from work) and much more!!

Types of Epidemics

Common Source Outbreak : One example of a common source outbreak is the epidemic of Emmititus, the disease reflected in the growth of the skull. In a common source outbreak, the affected individuals had an exposure to a common agent. If the exposure is singular and all of the affected individuals develop the disease over a single exposure and incubation course, it can be termed a point source outbreak. If the exposure was continuous or variable, it can be termed a continuous outbreak or intermittent outbreak, respectively.
Propagated Outbreak : In a propagated outbreak, the disease spreads person-to-person. Affected individuals may become independent reservoirs leading to further exposures. Many epidemics will have characteristics of both common source and propagated outbreaks. For example, secondary person-to-person spread may occur after a common source exposure or a environmental vectors may spread a zoonotic diseases agent.

Solution of Epidemics

We consider a model for single-season disease epidemics, with a delay (latent period) in the onset of infectivity and a decay (“quenching”) in host susceptibility described by time-varying rates of primary and secondary infections. The classical susceptible-exposed-infected (SEI) model of epidemiology is a special case with constant rates. The decaying rates force the epidemics to slow down, and eventually stop in a “quenched transient” state that depends on the full history of the epidemic including its initial state. This equilibrium state is neutrally stable (i.e., has zero-value eigenvalues), and cannot be studied using standard equilibrium analysis. We introduce a method that gives an approximate analytical solution for the quenched state. The method uses an interpolation between two exactly solvable limits and applies to the whole, five-dimensional parameter space of the model. Some applications of the solutions for analysis of epidemics are given.

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