History, ancient and modern, is full of plagues. The most famous one, perhaps, is the Black Death of the 1340's. The most recent one might be the 1918 influenza outbreak. Modern researchers speak of epidemics and pandemics, which are similar to a plague in the broad sense of the word. The word plague itself also has a narrower meaning, a specific disease: the Bubonic plague.
In the early part of the second century a plague broke out in the city of Antioch in Asia Minor (modern-day Turkey). A Roman solider dispatched a rescript to his superiors in Rome giving his report of the outbreak in which his writes that in light of the fact that the disease was spreading, the politicians, medical doctors and even family members had fled the city, and that he too found it necessary to pull his troops back to the periphery of the city to avoid contagion.
This solider then adds a curious detail: only one group remained in Antioch to tend to and bring comfort to the dying. It was, he reports, a sect of disciples of a man executed by his fellow Roman Pontius Pilate several decades previous. The solider found it incomprehensible that people would risk their very lives to bring comfort to the vulnerable and dying. These were early members of the sect of Christians.
In the Easter Letter from around 260 AD written by Dionysius, he indicates that while many Christians, especially the leaders, died, other Christians had survived the plague. Like most survivors, they were immune and were able to nurse many over the course of the plague. Others who had gained immunity by surviving the plague did not choose to care for the sick. This got the attention of the Roman officials. They desired that all citizens should demonstrate this concern.
Later, the Roman Emperor Julian, who hated these "Galileans", sought to get the pagans to imitate their care, but without success. Plagues reinforced faith rather than government edicts. Christianity embraced an new idea foreign to Roman mythologies; God expects his followers not only to worship him, but to care for others. This new religion, Christianity, demanded that its followers care for the sick, even the ailing Roman officials who had been involved with torturing and killing the Christians before the plague broke out. Caring for other humans beyond family ties, beyond business or political interests, as a religious requirement was a revolutionary idea.