I’ve noticed that the words ‘pandemic’ and ‘epidemic’ have recently been used interchangeably in the media and in everyday speech. Any extensive outburst of a contagious disease is a pandemic or an epidemic, depending on which prefix you prefer or comes to mind first. Pandemic sounds a bit more catastrophic, while epidemic has a scientific, even clinical, ring to it.
To do justice to the roots of the two words, it should be said that in an epidemic, the disease is in the process of spreading among the people, from the Greek epi- (among) and demos (people). A pandemic is when the disease has affected and infected everyone (from pan– meaning all). The myth of herd immunity can only work in a pandemic, when everyone’s infected, and then fingers crossed.
So a pandemic is not only far more catastrophic than an epidemic, but it’s also a point of no return – since everyone’s got the disease, there’s not much one can do in terms of prevention.
In the 4th century AD, a different classification was in use. The Roman historian Ammianus Marcellinus describes three types of outbreaks, what he calls endemic, epidemic and loemodemic or pestilential. The pandemic is absent from his triad, because Ammianus focused on the severity, not the contagion factor (what we now call the R0), of a disease.
Now the first kind of plague is called endemic, and causes those who live in places that are too dry to be cut off by frequent fevers. The second is epidemic, which breaks out at certain seasons of the year, dimming the sight of the eyes and causing a dangerous flow of moisture. The third is loemodes, which is also periodic, but deadly from its winged speed. (History, I, 489 (Rolfe edition))
By the 14th century, all plagues were referred to as pestilences. The Black Death was a pestilentia, as were countless other instances of outbreaks of smallpox, malaria and Yersinia Pestis-related plagues.
The only prefix which doesn’t come to mind yet is hypo- as in hypodemic, which describes a disease with a reproductive rate of less than 1. The day will come when Covid-19 goes into hypo-drive.