“Your character is suffering from a significant amount of Radiation poisoning. The more radiation damage, the more deadly the effect.”— In-game description
The character has a hidden radiation ("Rad") count that can be checked with a Geiger counter. This rad count causes the effect "radiated" to appear. As the Geiger count increases, further radiation poisoning occurs. Merely being "radiated" incurs no penalty. If the rad count gets high enough, however, SPECIAL stats begin to drop, and if any of these drop to zero due to poisoning, the character dies. Also, should the character be able to survive to maximum irradiation (1000 rads) (as in their stats do not reach zero), the character has 24 hours to use enough rad-aways to get themselves under 1000 rads or die anyway.