SPOLIER Why didn't

Not everyone reacts to radiation the same way. Some seem fine and get sick later. Others have symptoms right away (like Gen with his hair falling out) and recover. Gen also might not have received the same dosage that other people got. He probably just got lucky in how his body reacted and recovered.

Well, I am no expert on radiation sickness, but since this is an at least semi-autobiographical account of what actually happened to Keiji Nazakawa, who himself is portrayed as Gen, survived and lives today. So this must have really happened, and therefore it must be possible for people to have some symptoms and not others. You should watch a harrowing, although wonderful documentary called "white Light/Black Rain". Keiji speaks about his experiences as well as many other survivors. It is amazing.

Some who survived the nuclear fallout still lives today, actually I remember one of them was 90 years old according to discovery channel

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Hey soldier, do you know who's in command here?

http://en.wikipedia.org/wiki/Radiation_poisoning

This article gives a very good explanation about the different types of poisoning and how they can make a difference, examples of disasters, treatment, partial vs whole exposure (Gen's clothing would've offered a little more protection then just bare skin depending on thickness and material), and a neat table on exposure and symptoms with chances of fatality over how long (like 35% fatality over 30 days).

Essentially the soldier had a larger dose of more damaging radiations - probably due to a lack of cover at the precise moment of detonation. The most damaging radiation emitted by a nuclear device is the easiest to shield yourself from - Alpha particles, in particular, will be stopped by cloth not touching your skin or by a sheet of paper.

In Gen's case, he was stooped behind a wall when the device went off and thus shielded from the most damaging radiation. The milder case of radiation poisoning that he did get was probably absorbed from contact with people and objects that soaked up a much higher dose than he did.

Mild Radiation sickness is characterised by nausea and vomiting, with the duration from exposure to onset of the symptoms being a useful rule of thumb as to the severity of the exposure. These symptoms will usually occur for a few days after exposure and will be followed by the total recovery of the patient - however in the longer term even a mild dose of radiation sickness leads to an increased vulnerability to cancer.

Moderate Radiation sickness presents the same initial symptoms as a milder case, with the addition of internal bleeding, anemia and temporary hair loss. Moderate cases often present the 'walking ghost phase' where a patient recovers from the mild initial symptoms after a few days and then enjoys recovered health for a period of hours or days, depending upon dosage. Following the walking ghost phase the patient suffers a relapse, with the initial symptoms being accompanied by the more severe indicators given above. A temporary reduction in the white cell count in the patients blood will make him vulnerable to opportunistic infections.

Severe and Acute Radiation sickness presents the same initial symptoms as a milder case, but the walking ghost phase passes much more quickly and is followed by increasingly morbid symptoms as the radiation attacks and annihilates the patients bone marrow, white blood cell count, intestinal flora and inflicts systemic organ failures. Long term survival at this level of exposure is very unlikely, even if medical attention helps the patient survive the relapse symptoms there will usually be damage to the soft tissues causing leukemia and other cancers as well as organ failure.

Gen suffers a lower rate of exposure than his mother and the soldier, so he escapes the longer term effects of radiation sickness. I would classify his exposure as Mild.