Disasters are considered low probability but high impact events
Generally, they are defined by a overwhelming of the emergency department resources: the number of patients presenting to the department exceeds its capabilities.
Note that lack of supplies is rarely a problem in disasters taking place in populated regions.

Pre-Disaster: The events which are leading to the disaster are occurring, but often the fact that a disaster will occur is not obvious.
Prodromal Phase: The warning phase. The inevitability of the disaster becomes apparent. Steps may be made to decrease the disasters impact.
Impact Phase: The occurance of the event. May be short or prolonged. Often little can be done at this point to prevent the disaster from occuring
Rescue Phase: A time when immediate assistance can save lives. It is at this point that Emergency Medical Systems become overwhelmed.
Recovery Phase: Returning the population to normal. May take prolonged periods of time.
Disasters themselves can be further classified as Open vs Closed and Finite vs Ongoing.
In general, at the disaster site itself, physicians are no better able to provide emergency care than are paramedics. Some reports have indicated that physicians and nursed at the disaster site have no effect on victim mortality.
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