The acute radiation syndrome (ars) is also known as radiation sickness. A person exposed to radiation will develop ars only if the radiation dose was high, penetrating (e.g., x-rays or gamma rays), encompassed most or all of the body, and was received in a short period of time. Clinical severity of the four subsyndromes of ars (hematopoietic, cutaneous, gastrointestinal, and neurovascular) will vary with dose and host factors (e.g., young or old age, immunosuppression, and medical co-morbidity--especially extensive trauma and burns).