Contagion and a lack of understanding of disease vectors, combined with the related inability to treat them, were all factors contributing to illnesses becoming fatal. For newly introduced diseases, lack of immunity was a primary factor in rapid spread of diseases. The high population density in the largest cities, including Athens and Rome, as well as unsanitary conditions, exacerbated the spread. Once infection rates reached epidemic proportions, curing was impossible.
Even common problems such as parasites and related gastrointestinal diseases could be fatal when left untreated. The combination of rapidly depleted populations in the cities and the spread through the rural work force both contributed to weakening the empire’s people. Military and political expansion were curtailed, and the centers could not sustain their web of control over the outlying areas. Loss of faith in the leaders also undermined political stability, sometimes leading to rebellions or coups.
In ancient Greece, Hippocrates compiled records of diseases. Among the most dangerous ones were cholera, meningitis, and tuberculosis. The Great Plague that affected Athens from 430 to 428 BCE is likely to have been bubonic plague. Sophocles also described a plague in Oedipus Rex. Hippocrates noted severe respiratory distress that was often fatal; this was probably influenza.
The primary potentially fatal diseases that affected ancient Rome were malaria, plague, and tuberculosis; leprosy was probably present as well. Because of the wide-ranging trade in the empire, ships from distant ports routinely docked in Rome. Malaria most likely spread from North Africa via Sardinia. Infected passengers or water carrying mosquito larvae could have brought the disease.
What is known as plague, such as the Antonine Plague of the second century BCE, may have been smallpox. At its worst, it apparently took up to 2,000 lives a day in Rome. In the sixth century AD, the plague reported may have been bubonic plague.
Grmek, Mirko. 1991. Diseases in the Ancient Greek World. Baltimore: Johns Hopkins University Press.
Harper, Kyle. 2017. The Fate of Rome: Climate, Disease, and the End of an Empire. Princeton: Princeton University Press.