Contagious disease

These diseases are caused by organisms such as parasites, bacteria, fungi, and viruses. Many types of organisms live on the human body and they can sometimes cause disease.

Some common infectious diseases known to be contagious are influenza, COVID-19, ebola, hepatitis, HIV/AIDS, Human papillomavirus infection, Polio, and Zika virus.

A disease is often known to be contagious before medical science discovers its causative agent. Koch's postulates, which were published at the end of the 19th century, were the standard for the next 100 years or more, especially with diseases caused by bacteria. Microbial pathogenesis attempts to account for diseases caused by a virus.

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 emphasize very infectious, easily transmitted, or especially severe communicable diseases.

In 1849, John Snow first proposed that cholera was a contagious disease.