Disasters are commonly categorized by their origin; natural or man-made. Most disasters investigated in the literature are natural disasters. Recently, however, industrial accidents have been categorized as disasters. The Bhopal gas release and the Che rnobyl nuclear accident are two examples of a man-made disaster. Forest fires (initiated by man) may be another example.

Disasters may occur suddenly in time (a quick onset), or they may develop over a period of time (a slow onset). Most occur suddenly and perhaps unexpectedly. However, some events develop gradually, including some floods and famines related to drought.