Two plagues devastated the Roman empire during the third century. The Antonine Plague (ca.165-180) ended roughly 75 years before Perilous Privilege begins. The Plague of Cyprian (ca. 250-270) ran through Gallienus’ entire reign.
Between five and ten million people died from Antonine Plague. No death estimates are available for Plague of Cyprian, however, historical sources state that five-thousand people a day died in Rome at height of the latter.
Mortality rates were especially high where people were close together, notably the army and cities. In the countryside, men who hadn’t died were conscripted in the army or killed by enemy raids. As a result fields went untended causing food shortages. Commerce suffered; the economy declined; social instability resulted.
Plagues were not an issue during the period of hunter-gatherers. There were too few of them to effectively transmit these diseases. That changed when humans began growing crops and raising animals. This enabled much higher population densities to evolve—from villages to towns to cities—and brought humans into close contact with domesticated animals.
Plagues discussed here (Influenza, Smallpox, Measles) are caused by parasitic viruses that require a host for reproduction. Their collective survival depends on their ability to leave a host and infect a new host, since infected hosts either died or developed immunity to further infections. In order for the disease to spread, people must live closely together.
Birds are natural reservoirs for a number of viruses. They carry the virus but are immune to it. Ducks and chickens became domesticated as well as pigs. Pig cells happen to share some of the same features as both birds and humans. After many natural mutations, viruses developed an ability to spillover from animals to humans.
High rates of infection for both plagues suggest that they were different types and that neither had infected these populations before. Romans believed the diseases were caused by the wrath of the gods. So much of their approach to treating these afflictions was through prayers and sacrifices.
A similar lack of natural immunity existed when Europeans first brought infectious diseases to the New World. Despite our understanding of infectious diseases and far more advanced medical treatments, we are still susceptible to new epidemics, COVID being the most recent example.
References
Pale Rider, The Spanish Flu of 1918 and How it Changed the World, Laura Spinney, 2017.
Wikipedia: Antonine Plague
Wikipedia: Plague of Cyprian
