The average person in the US is exposed to background radiation levels that would result in an annual dose of approximately 360 mrem.  Higher and more short-term doses, although unlikely, are termed acute exposures.  Whole-body doses of radiation (of the type in x-ray or gamma radiation) in significant doses of 35 rad can cause nausea, weakness and appetite loss within a few hours following an acute exposure.  These symptoms will disappear within a few hours of the exposure.  At doses between 125 – 300 rad, there is increasing likelihood of severity of nausea, vomiting and weakness with symptoms persisting for up to two days.  There is 50% mortality from acute exposures greater than 350 rad without medical treatment. 

Infection is the main cause of death after irradiation.  However, cells differ in their sensitivity to ionizing radiation damage. After exposure, many sufficiently healthy cells are used up defending the body from infection, and others are prevented from performing their duty.  Virtually no new replacement cells are produced because of the extensive damage to stem cells in bone marrow.