# What is he difference between a contagious and infectious?

**URL:** <https://boards.straightdope.com/t/what-is-he-difference-between-a-contagious-and-infectious/810472>\
**Category:** Factual Questions\
**Created:** [March 13, 2018, 5:47am UTC](https://boards.straightdope.com/t/what-is-he-difference-between-a-contagious-and-infectious/810472 "2018-03-13T05:47:39Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![Steven\_Estes](https://avatars.discourse-cdn.com/v4/letter/s/94ad74/32.png) [@Steven\_Estes](https://boards.straightdope.com/u/Steven_Estes)\
**Post date:** [March 13, 2018, 5:47am UTC](https://boards.straightdope.com/t/what-is-he-difference-between-a-contagious-and-infectious/810472/1 "2018-03-13T05:47:39Z")

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What is he difference between a contagious and infectious?  
Dictionaries do not make this clear, but perhaps it is because there is overlap in the meanings.

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**Author:** ![DavidwithanR](https://avatars.discourse-cdn.com/v4/letter/d/3bc359/32.png) [@DavidwithanR](https://boards.straightdope.com/u/DavidwithanR)\
**Post date:** [March 13, 2018, 5:59am UTC](https://boards.straightdope.com/t/what-is-he-difference-between-a-contagious-and-infectious/810472/2 "2018-03-13T05:59:27Z")

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You’re right, the meanings overlap and the difference is not always clear. For every clear and unambiguous answer as to “the real difference”, there will be a response showing why it isn’t that simple.

“Contagious” generally means transmission is directly from one human to another human - no mosquito or other “middleman”.

One writer on Wikipedia suggests that “contagious” often turns out to mean that quarantine would be a reasonable and useful response.

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**Author:** ![UDS](https://avatars.discourse-cdn.com/v4/letter/u/ecae2f/32.png) [@UDS](https://boards.straightdope.com/u/UDS)\
**Post date:** [March 13, 2018, 6:07am UTC](https://boards.straightdope.com/t/what-is-he-difference-between-a-contagious-and-infectious/810472/3 "2018-03-13T06:07:14Z")

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In the medical context, “contagious” is a subset of “infectious”.

Contagious diseases are infectious by contact - i.e. you can be infected by the disease through contact with someone who is themselves infected. A disease which is spread through, e.g. contaminated water supply, or infected mosquitos or other insects or animals, would be infectious but not contagious.

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**Author:** ![guizot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/guizot/32/3636_2.png) [@guizot](https://boards.straightdope.com/u/guizot)\
**Post date:** [March 13, 2018, 6:22am UTC](https://boards.straightdope.com/t/what-is-he-difference-between-a-contagious-and-infectious/810472/4 "2018-03-13T06:22:38Z")

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[Here’s a comparison](https://s17.postimg.org/b0lekjq4f/diseases.png) of how the two words are used as attributive adjectives.

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**Author:** ![John\_DiFool](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_difool/32/19543_2.png) [@John\_DiFool](https://boards.straightdope.com/u/John_DiFool)\
**Post date:** [March 13, 2018, 12:18pm UTC](https://boards.straightdope.com/t/what-is-he-difference-between-a-contagious-and-infectious/810472/5 "2018-03-13T12:18:27Z")

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Adjectiving weirds language.

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**Author:** ![Qadgop\_the\_Mercotan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/qadgop_the_mercotan/32/83_2.png) [@Qadgop\_the\_Mercotan](https://boards.straightdope.com/u/Qadgop_the_Mercotan)\
**Post date:** [March 13, 2018, 1:18pm UTC](https://boards.straightdope.com/t/what-is-he-difference-between-a-contagious-and-infectious/810472/6 "2018-03-13T13:18:58Z")

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A contagious infection can be spread to directly to others. Like colds, flus, active tuberculosis.

But many infections are not contagious. Urinary tract infections, sinus infections, ear infections.