thumb|upright=1.8|<div style="text-align: center;">Diagram of the causes of mortality in the army in the East, F. Nightingale, 1858</div>
Zymotic disease was a 19th-century medical term for acute infectious diseases, especially "chief fevers and contagious diseases (e.g. typhus and typhoid fevers, smallpox, scarlet fever, measles, erysipelas, cholera, whooping-cough, diphtheria, etc.)".
Zyme or microzyme was the name of the organism presumed to be the cause of the disease.
<blockquote>As originally employed by William Farr, of the British Registrar-General's department, the term included the diseases which were "epidemic, endemic and contagious," and were regarded as owing their origin to the presence of a morbific principle in the system, acting in a manner analogous to, although not identical with, the process of fermentation.
The word zymotic comes from the Greek word ζυμοῦν zumoûn which means "to ferment". It was in British official use from 1839. This term was used extensively in the English Bills of Mortality as a cause of death from 1842. In 1877, Thomas Watson wrote in a Scientific American article "Zymotic Disease" describing contagion as the origin of infectious diseases.
Robert Newstead (1859–1947) used this term in a 1907 publication in the Annals of Tropical Medicine and Parasitology, to describe the contribution of house flies (Musca domestica) towards the spread of infectious diseases. However, by the early 1900s, bacteriology "displaced the old fermentation theory",