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A contagious disease (or communicable disease) is a disease that readily spread (that is, communicated) by transmission of a pathogen from an infected person to another person. Contagious diseases vary in how readily they are communicated. For example, COVID-19, which spreads by transmission of the SARS-CoV-2 coronavirus from one person to another, is extremely contagious, as evidenced by the global pandemic it caused.
Conversely, a non-contagious disease either cannot be transmitted from one person to another or the probability of transmitting the disease to another person is low.
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. Microbial pathogenesis attempts to account for diseases caused by a virus.
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 emphasise very infectious, easily transmitted, or especially severe communicable disease.
In 1849, John Snow first proposed that cholera was a contagious disease.
Effect on public health response
Most epidemics are caused by contagious diseases, with occasional exceptions, such as yellow fever. 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 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.[failed verification]
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.
- "Merriam-Webster dictionary definition of contagious disease". Retrieved 2009-11-27.
- Non-Contagious Diseases - Contact With www.transfusionguidelines.org, accessed 27 January 2020.
- A primer from the CDC on quarantine and its uses against contagious disease spread Accessed Nov. 27, 2009.