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On Oxford Advanced Learner's Dictionary we read:

Infectious: an infectious disease can be passed easily from one person to another, especially through the air they breathe.

Contagious: if a person is contagious, they have a disease that can be spread to other people by touch.

But, cholera and typhoid (for example) are usually spread through the environment.

I am not sure what is the word to use on such cases. Could we say contagious in cholera disease?

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OED says yes –  Em1 Apr 13 '12 at 14:03
    
@Em1 - Thank you for the reference. –  user19148 Apr 13 '12 at 14:46
    
One more –  Em1 Apr 13 '12 at 14:47

5 Answers 5

up vote 6 down vote accepted

Contagious is defined as "capable of being transmitted from individual to individual (person to person; animal to animal); communicable."

Infectious is defined as "caused by or capable of being communicated by infection." And to understand that you need to know that infection is defined as "1. invasion and multipication of microorganisms in body tissues."

So something can be infectious without being contagious. Cholera and typhoid fever are both termed water- and food-borne infectious diseases. (They are caused by bacteria and spread through contaminated water and food.) So, no, I would not say cholera is contagious, but it is an infectious disease.

Examples of contagious disease are AIDS, influenza, and the common cold, which are passed fairly easily through contact between an infected individual and a susceptible individual.

(Sources: Dorland's Illustrated Medical Dictionary, 30th edition, and Saunders Comprehensive Veterinary Dictionary, 2nd edition.)

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A disease is infectious; a person with that disease may be contagious.

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Technically, no, a person cannot be contagious (adjective meaning "capable of being transmitted from one individual to another"). A person may have a contagious disease. –  JLG Apr 13 '12 at 15:23
    
@JLG: You may be technically correct (I am not a doctor, nor do I play one on the internet), but I believe my definitions both reflect customary use in American English. –  Adam Musch Apr 13 '12 at 16:09
    
@JLG: Also, note "if a person is contagious", evidently quoted from Oxford Advanced Learner's Dictionary in the OP's question body. –  John Y Apr 13 '12 at 18:03
    
@John Y, I noticed that in the question and realize that non-medical dictionaries have a definition that means contagion-bearing. –  JLG Apr 13 '12 at 18:14

The NOAD has the following notes about the usage of contagious and infectious:

Strictly, a contagious disease is one transmitted by physical contact, whereas an infectious one is transmitted via microorganisms in the air or water. In practice, there is little or no difference in meaning between contagious and infectious when applied to disease or its spread. In figurative senses, contagious may describe the spread of good things such as laughter and enthusiasm or bad ones such as violence or panic, whereas infectious usually refers to the spread of positive things, such as good humor or optimism.

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Speaking from my intuition here (which includes some formal education in biology):

  • An infection occurs when pathogenic organisms such as bacteria or fungi cause damage to your tissues and physiology.
  • A contagion occurs when those organisms are transmitted (over the air or other media) to other individuals, where they may cause new infections.

In a nutshell, infection is the nasty consequence of having the bugs; contagion is their transmission.

In practice, most diseases caused by infection are also contagious, so these two concepts are sometimes mixed up. But it doesn't need to be the case. Some infectious diseases, especially during some specific stages of the infection, cease being contagious altogether. Tertiary syphilis is an example.

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So contagion doesn't need contact between infected individuals? –  Andrew Leach Apr 13 '12 at 14:54
    
No, it doesn't need physical contact. Contagion can happen over the air, fluids, shared towels or other utensils, etc. –  CesarGon Apr 13 '12 at 23:31

This is one of many cases (like "is a tomato a fruit or a vegetable?") where the answer is different depending on whether the context is technical or everyday.

Technically, certainly contagion is different from infection, and a disease may be infectious without being contagious.

In ordinary speech, most people make little or no distinction between the two.

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Probably true, but the OP asked about two specific diseases and asked for the distinction between the two terms. –  JLG Apr 13 '12 at 16:35

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