There are multiple ways to approach any topic. You won't find all the information in one place or under one subject search. You have to get information from many sources and put them together. From an encyclopedia article or from searching in the library catalog, collect a list of words and names that could be relevant. Combine the terms in various ways. Expand the list as you explore and learn.
Example: Epidemics in Ancient Rome
Explore these subjects, names, and keywords:
When you find a book or article that's really useful, look at the subjects assigned to it in the library catalog. If there are more items on that topic, the subject will be hot-linked to them - just click to get a new list.
Note authors who keep appearing in your searches - they are probably the leading experts in the field. You can search their names in the catalog to find more of their books.