The theory that certain diseases can be transmitted by contact between a person carrying an infection and a susceptible host is solidly based in empirical observations that date back at least three millennia. In the Bible, mention is made of lepers being required to wear distinctive clothing and carry a bell to warn others of their presence, a practice that implies contagion as the cause of leprosy. The Bible also describes the use of lazarettos, where people with what were believed to be contagious diseases could be incarcerated. Persons suffering from diseases believed to be contagious have been isolated and shunned in this way for thousands of years. Tuberculosis and other diseases, including some that are not contagious but can be disfiguring, such as psoriasis, have long carried a social stigma.
The concept of contagion was described in De Contagione et Contagiosis Morbis (1546) by Hieronymus Fracastorius (1478–1553). He distinguished three types of contagious disease: direct (spread by person-to-person contact); droplet (spread by, for instance, sneezing, coughing); and by way of contaminated clothing, cooking utensils, and other items. Such objects that harbor disease agents are known as fomites.
The concept of contagion antedates the germ theory of disease and the first microscopic observations by Antoni van Leeuwenhoek (1632–1723) of tiny living creatures, later called germs or microbes, that were shown by Robert Koch (1843–1910) and others to be the causal agents for many diseases. Nowadays the word "contagious" is usually reserved for diseases that are both dangerous and highly infectious. Most other transmittable diseases are simply called either communicable, or infectious. Diseases referred to as infectious are sometimes considered more readily transmitted to others than diseases that are communicable, but usage of these words is loose and inconsistent. It is ironic that leprosy, the most feared contagious disease of antiquity, is actually among the least communicable; it is transmitted by a bacillus that behaves so sluggishly it requires prolonged close contact for transmission to occur.
JOHN M. LAST