The effect of radiation depends on the dose and also how it is delivered. A dose delivered at once is more dangerous than one spread out over a long period of time. Also one delivered to the whole body is far more dangerous than one directed at a small area that is radiosensitive (e.g. bone marrow), although this is more dangerous than radiation directed at a radioresistant part (e.g. muscles). Low doses are used commonly in medicine.
Doses below 100rems do not produce radiation sickness, although there may be some temporary changes to the blood-cell count. Doses of 100 to 200rems produce vomiting and nausea, there is also a decrease in the number of whiter blood cells, although the victim will recover in a few weeks.
Doses of 200 to 600rems lead to severe radiation sickness, causing a severe reduction in white blood cells, internal hemorrhages, infection and loss of hair. With 0-80% having a lethal outcome in 2 months. 600 - 1000rems is similar but causes much more severe damage, 80-100% dying within 2 months.
Doses greater than 1000rems are usually fatal within 2 weeks, as the lining of the intestine is destroyed causing diarrhea, fever and an electrolyte imbalance. 5000rems or more result in cerebral death by direct damage to the brain, causing convulsions, tremor, lack of coordination and lethargy, death is usually within 2 days.