The Plague Dragons don't do anything as nice as spread disease or famine, they ravage the land during their mating cycle, destroying property, buildings, even affecting entire lakes and rivers with the destructive nature of their courtship and mating.
The cycle begins with a Queen Plague Dragon making her lair deep in the earth where the temperate is stable. The injuries the Queen sustains during mating often results in her death, and her decaying corpse provides her clutch of eggs with the correct temperatures to encourage a new generation of strong male Plague Dragons and a new Queen Plague Dragon for the next cycle to renew. Some stronger Queens will survive, and tend to their clutch during the several hundred year incubation period. These Alpha Queens will often seek out the queen egg and either cast it out of the nest to die, or actively smash it to keep a new queen from competing for a mate.
After the incubation period is over, the times the prophesies warn of occur: the new generation of Plague Dragons hatch. After breaking free of their shell, they have to break through a second shell of rock to emerge and look for a new mate. After a year of hunting and eating anything it can, the young hatchling begins to groom a territory suitable for a Queen to mate with, and gifts to entice her. Although there are some exceptions, Plague Dragons typically like gifts that are meaty or shiny, or both in the case of a paladin. Once a Queen has chosen her mate, she will defend her mate and territory until she is ready to mate, or is presented with a more enticing gift. The mate dragons will often fight and kill each other over territory or gifts for the Queen.
Once the courtship is over and the Queen Plague dragon has a clutch of eggs ready, the dragons will mate, and then the Queen will try to kill and eat the male, mantis style. If a male can make it away without dying, he may remain in his territory and set up a lair to hoard gifts for the next generations queen and further tend his territory. With mating and murder over, the Queen will use the territory available to dig a lair for her clutch. This often starts in the soft soil of a swamp, or a river or creek bed. Not only does this make the initial digging easier, it also conceals the lair's location from outside forces.
Most Plague Dragons only survive two or three generations, but they have no known age limit and no signs of aging have been observed
The time draws nigh ...
Recent small earthquakes point to the return of dragons, and the PCs have to help evacuate towns, or battle the young hatchlings before they become too powerful.
They dug too deep...
A dwarven mine discovered a Queen Plague Dragon lair. She's alive and protecting her clutch of natural disasters, and the dwarves want her gone.