2. Diseases which persist when a population is dense enough to keep transmitting germs and big enough to keep producing new susceptibles: Epidemiologists say that COVID-19 belongs to the illnesses termed zymotics, which are severe illnesses that can cause death.
A couple of examples of the word zymotics being used in prior writings
Small-pox is one of a group of allied diseases, called the Zymotics. The name means that the disease is due to a process of fermentation.
For common-sense purposes, it is better to call these diseases by the plain English name of filth diseases.
They are diseases which take their rise in filth, which are nature's punishment for filth, which are both frequent and virulent where filth prevails, and which can be cleared away by the clearing away of filth.
Now, in the eighteenth century, in the latter part of which Jenner lived, it must be confessed that the English people had not yet awakened to the beauty and the necessity of cleanliness.
Filth was universal, and small-pox was terrible. Not so terrible as many people want to make out, but still a formidable danger.
"Only when a community is dense and filthy enough to keep spreading germs and big enough to keep supplying new susceptibles do such infections as measles, smallpox, typhoid, and influenza crowd diseases or zymotics stay in circulation."