Ionizing radiation tears the cellular structure apart. The skin blisters, turns red, then black. This is followed by a latency period. The immediate effects subside, the patient appears to be recovering. Healthy, even, but they aren't. This usually only lasts for a day or two.
The bone marrow dies. The immune system fails. The organs and soft tissue begin to decompose. The arteries and veins spill open like sieves, to the point where you can't even administer morphine for the pain, which is... unimaginable. Within three days to three weeks, you're dead.