A disaster is an event causing damage or hardship. The word comes from Greek and Latin terms suggesting "from the stars" and hence has a connotation of bad luck: "Lillian felt that the loss of all her money was a minor disaster." "The collision of the cars was a disaster, but not a fatal one." Holocaust comes from a Greek word meaning "burnt" or "burned" and refers to complete devastation or destruction. That is, a holocaust is a disaster, but a disaster is not necessarily a holocaust. Floods, collisions, train wrecks, and accidents of many kinds are disasters, but holocaust is a term that should be reserved for immense destruction and widespread devastation: "The bombs falling in the crowded area resulted in a holocaust." Tragedy, a more general term, refers to any calamity or disaster, any dreadful or fatal event. One may, for instance, refer to the disaster or holocaust or tragedy of modern warfare.
Dictionary of problem words and expressions. Harry Shaw. 1975.