Serious illness caused by being exposed to high doses of certain types of radiation, usually over a short period of time. Symptoms of radiation sickness usually occur right after exposure but they may happen over time, and they may come and go. Symptoms include nausea and vomiting, diarrhea, headache, dizziness, weakness, fatigue, bleeding, hair loss, swelling, itching, and redness of the skin, and other skin problems. Very large doses of radiation may cause death. Also called acute radiation sickness, acute radiation syndrome, radiation poisoning, and radiation sickness syndrome.