It's caused by...well, radiation. If you go over a certain amount of REM's (Roetgen Equiv. in Man), you can start getting different symptoms, since radiation is cumulative, it builds up over time. If you work in a facility, you're more than likely to be taken out of there before you pass the legal limit set by the NRC. Nuclear war or an accident like Chernobyl is different. But dude, you do NOT want to know what the symptoms of radiation sickness or the affects or pictures look like. Dramatizations are nothing like it. First, your hair and teeth start falling out and hemmoraging and mutation and...ugh. I don't even want to go into it. It's really sick stuff, and if you live thru it, you will have a really high chance for cancer and genetic mutation if you have offspring.