You searched for: “nosology
1. The science of the classification of diseases.
2. The study of diseases and, in particular, their classification, grouping, ordering, and relationship to one another; it includes the formulation of principles for differentiating one disease from another.
3. The classification of ill people into groups, whatever the criteria for the classification, and agreement as to the boundaries of the group.

Pointing to a page about human diseases or nosology. When diseases started to become plagues is explained at this nosology page.

(diseases spread as mankind congregated into a squalor of cities)