Radation is harmful because its energy is higher than the engery in chemical reactions. The effects of radiation range from hair, brain, thyroid, the blood system, heart, Gastrointestinal tract, and Reproductive tract.
Hair - the loss of hair occurs with exposure to radation at 200 rems or higher
Brain - brain cells do not reproduce and radiation exposure of 5,000 rems or higher can damage brain cells directly. The radiation can cause seizures and immediate death
Thyroid - radioactive exposure can destroy part of or all of the thyriod
Blood System - radioactive exposure to 100 rems, the blood's lymphocyte cell count will be reduced. This will leave the victim more susceptible to infection  
Heart - radioactive activity exposure of 1,000 to 1,500 rems causes damage to the small blood vessels and can cause heart damage and immediate death
Gastrointestinal tract - radioactive exposure can damage to the intestinal tract lining will cause nausea, bloody vomiting and diarrhea. This occurs at exposure of 200 rems.
Reproductive tract - radioactive exposure of 200 rems can cause the victim to become sterile.