A contagious disease is a subset category of transmissible 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. Contagiousness varies between diseases. Non-contagious infections, by contrast, usually require a special mode of transmission between persons or hosts. These include need for intermediate vector species or by non-casual transfer of bodily fluid. 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 infections are considered contagious, but only some of them are subject to medical isolation. A disease may be known to be contagious but its causative factors remain undetermined. A contagion may be more infectious if the incubation period is long.
Historical meaning
Originally, the term referred to a contagion 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.
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. 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.
