Plague was not a myth - it was a reality that could destroy entire villages and kill thousands of people within a very short timeframe. By far, the most famous plague is the Black Death, which occurred in 1347 and ended in 1350.
This, along with invasions, was the biggest fear villagers suffered in the Dark Ages. Fathers lost their sons - and wives their husbands. Plague led to a major instability across Europe when a major outbreak swept a good percentage of the population.
The Black Death
The Black Death was a plague that killed one third of the European population. It began in 1347 and ended in 1350 - though many other incidents of death occurred thereafter. It is impossible to certainly know what the Black Death did, or where it was originated. According to many scientists, it is supposed that the Black Death was carried by rats coming from the East (China).
This is by far the most famous plague since it killed so many people. "Brothers deserted brothers" - "The cattle was moving unattended." It was such the hysteria formed through the Black Death that many people even today fear for something similar to happen in the future.