As a general rule, the more people in the smaller space, the greater the chance and extent of an outbreak. One of the earliest plagues known was the European bubonic plague in the 14th century.
In today's globalizing world and blossoming population, the change of a serious outbreak is becoming more and more real. Current outbreaks--rather in the silent--include AIDS, other venereal diseases, flu, cholera, malaria, typhus, and the four hemorrhagic fevers (which includes ebola). The recent avian flu scare is but merely an extension of this trouble.
Plagues cannot be eradicated without a concerted worldwide effort. But they CAN, and they have been: smallpox has now been virtually eradicated from the world.
The threat of even more dangerous, more deadly, more insidious plague looms in the future. Whether natural, or artificial, the threat is becoming more and more real.