Radiation poisoning

Fallout 3
Radiation Poisoning occurs when the player's radiation level passes a certain threshold. Radiation poisoning has three levels, minor radiation poisoning, advanced radiation poisoning and fatal radiation poisoning. During radiation poisoning, the player will lose certain skill points. Reaching 1000 radiation points results in the player's death.

Your radiation level increases in several ways: when you eat certain meats, E.g. Radroach meat, Mole rat meat etc. When you eat certain types of food, E.g. pork and beans, sugar bombs etc. When you swim in or drink contaminated water. Or when you get too close to a contaminated area. E.g. Megaton after you blow it up.