A communicable disease is a disease that readily spread (that is, communicated) by transmission of a pathogen from an infected person to another person. communicable diseases vary in how readily they are communicated. For example, COVID-19, which spreads by transmission of the SARS-CoV-2 coronavirus from one person to another, is extremely contagious.

Conversely, a non-contagious disease either cannot be transmitted from one person to another or the probability of transmitting the disease to another person is low.

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.
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