A contagious disease is a subset category of transmissible diseases, usually infections or some non-infection diseases, which are transmitted to other persons, either by physical contact with the person suffering the disease, or by casual contact with their secretions or objects touched by them or airborne route among other routes.
Non-contagious infections, by contrast, usually require a special mode of transmission between persons or hosts. These include need for intermediate vector species (mosquitoes that cause malaria) or by non-casual transfer of bodily fluid (such as transfusions, needle sharing or sexual contact). They can also be inherited from parents or caused by environmental or behavioral factors.
The boundary between contagious and non-contagious infectious diseases is not perfectly drawn, as illustrated classically by tuberculosis, which is clearly transmissible from person to person, but was not classically considered a contagious disease. In the present day, most sexually transmitted diseases are considered contagious, but only some of them are subject to medical isolation.
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.
Most epidemics are caused by contagious diseases, with occasional exceptions, such as black plague. The spread of non-contagious communicable diseases, such as yellow fever or filariasis, is little or not affected by medical isolation (for 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.
- Merriam-Webster dictionary definition of contagious disease. Accessed Nov. 27, 2009
- http://www.encyclopedia.com/doc/1O62-contagiousdisease.html Definition from nursing encyclopedia.
-  A primer from the CDC on quarantine and its uses against contagious disease spread. Accessed Nov. 27, 2009.