Demographic Crises
Forms of Plague
Bubonic plague is the most common form, though not the most virulent. It is a bacillus, or rather a family of bacilli, that lives in insects and rodents. It appears to originate in the insects and most relevant it lives in fleas. When the bacilli multiply too greatly, they block the flea's stomach and it regurgitates the contents into a host while feeding. That's how the disease gets into rodents. While the disease normally stops there, sometimes causing die-offs in a rodent population, once in a while the disease will jump over to another flea that feeds on humans.
Why these changes happen is not at all clear. To be more precise, while we are gaining a better and better understanding of the disease in modern times, we cannot be sure this is the way it behaved eight hundred years ago when both the climate and the ecology were different.
It's worth noting that while the common black rat has taken the majority of the blame for the spread of the plague (four black rats circle ominously on the cover of my copy of Gottfried's book on the Black Death), the disease can be carried by many rodents and also by many domesticated animals, including pigs, sheep, dogs and cats—pretty much everything you'd find around human habitation with the odd exception of the horse.
Bubonic plague is usually fatal, though not inevitably so. Today, we have drugs that can cure it, if administered in time. But if the victim is already at risk, through malnutrition or other illness, it is more deadly. There were plenty of people in the 1340s who were at risk.
There are two other varieties of plague: septicaemic plague, which attacks the blood, and pneumonic plague, which attacks the lungs. The latter is especially dangerous as it can be transmitted through the air. When we read in the accounts of the spread of the disease "by sight and breath" we are probably reading about pneumonic plague. Pneumonic plague flourishes in cold, damp weather and not in hot, dry weather. A characteristic of septicaemic plague is an extremely swift death, even within hours. Death comes so quickly, the boils do not have time to form and the only outward sign is a rash. Both these two are nearly 100% fatal.