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A contagious disease is an infectious
disease A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that a ...
that is readily spread (that is, communicated) by transmission of a
pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a g ...
through contact (direct or indirect) with an infected person. A disease is often known to be contagious before medical science discovers its causative agent. Koch's postulates, which were published at the end of the 19th century, were the standard for the next 100 years or more, especially with diseases caused by
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...
. Microbial pathogenesis attempts to account for diseases caused by a
virus A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsk ...
. The disease itself can also be called a contagion.


Historical meaning

Originally, the term referred to a ''contagion'' (a derivative of 'contact') or disease transmissible only by direct physical contact. In the modern-day, the term has sometimes been broadened to encompass ''any'' communicable or infectious disease. Often the word can only be understood in context, where it is used to emphasize very infectious, easily transmitted, or especially severe communicable diseases. In 1849, John Snow first proposed that cholera was a contagious disease.


Effect on public health response

Most
epidemics An epidemic (from Greek ἐπί ''epi'' "upon or above" and δῆμος ''demos'' "people") is the rapid spread of disease to a large number of patients among a given population within an area in a short period of time. Epidemics of infectiou ...
are caused by contagious diseases, with occasional exceptions, such as
yellow fever Yellow fever is a viral disease of typically short duration. In most cases, symptoms include fever, chills, loss of appetite, nausea, muscle pains – particularly in the back – and headaches. Symptoms typically improve within five days. ...
. The spread of non-contagious communicable diseases is changed either very little or not at all by medical isolation of ill persons or medical
quarantine A quarantine is a restriction on the movement of people, animals and goods which is intended to prevent the spread of disease or pests. It is often used in connection to disease and illness, preventing the movement of those who may have been ...
for exposed persons. Thus, a "contagious disease" is sometimes defined in practical terms, as a disease for which isolation or quarantine are useful public health responses.A primer from the CDC on quarantine and its uses against contagious disease spread
Accessed Nov. 27, 2009. Some locations are better suited for the research into the contagious pathogens due to the reduced risk of transmission afforded by a remote or isolated location. Negative room pressure is a technique in health care facilities based on aerobiological designs.


See also

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Germ theory of disease The germ theory of disease is the currently accepted scientific theory for many diseases. It states that microorganisms known as pathogens or "germs" can lead to disease. These small organisms, too small to be seen without magnification, invade ...
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Herd immunity Herd immunity (also called herd effect, community immunity, population immunity, or mass immunity) is a form of indirect protection that applies only to contagious diseases. It occurs when a sufficient percentage of a population has become im ...


References

{{Authority control Infectious diseases Microbiology Diseases and disorders Epidemiology Causality