As the author of the Persian book I’m currently translating mentions a section in Ibn Khaldun’s “Al-Muqaddimah,” I checked both the original “Al-Muqaddimah” in Arabic and its English rendition by Franz Rosenthal (https://amzn.to/2yUHAvX) for verification and knowing the context.
Also known as “Al-Muqaddimatu Ibn Khaldūn” (Ibn Khaldūn’s Prolegomena), “Al-Muqaddimah” is a book written by the Arab polymath Ibn Khaldun (https://amzn.to/2V0weih) in 1377 which records an early view of universal history, and intended as the introduction chapter and the first book of his planned work of world history, “Kitāb al-ʻIbar” (Book of Lessons), but already regarded as an independent work on its own even during his lifetime.
As I was browsing subsequent pages of the book, this section on the occurrence of pestilences inadvertently caught my attention:
“In the later (years) of dynasties, famines and pestilences become numerous… The large number of pestilences has its reason in the large number of famines just mentioned. Or, it has its reason in the many disturbances that result from the disintegration of the dynasty. There is much unrest and bloodshed, and plagues occur. The principal reason for the latter is the corruption of the air (climate) through (too) large a civilization (population). It results from the putrefaction and the many evil moistures with which (the air) has contact (in a dense civilization). Now, air nourishes the animal spirit and is constantly with it. When it is corrupted, corruption affects the temper of (the spirit). If the corruption is strong, the lung is afflicted with disease. This results in epidemics, which affect the lung in particular. (Even) if the corruption is not strong or great, – putrefaction grows and multiplies under (its influence), resulting in many fevers that affect the tempers, and the bodies become sick and perish. The reason for the growth of putrefaction and evil moistures is invariably a dense and abundant civilization such as exists in the later (years) of a dynasty… Therefore, it has been clarified by science in the proper place that it is necessary to have empty spaces and waste regions interspersed between civilized areas. This makes circulation of the air possible. It removes the corruption and putrefaction affecting the air after contact with living beings, and brings healthy air. This also is the reason why pestilences occur much more frequently in densely settled cities than elsewhere… (Ibn Khaldun, “Al-Muqaddimah: An Introduction to History,” trans. Franz Rosenthal (New Jersey: Princeton University Press, 2005), p. 256, https://amzn.to/2yUHAvX).