Ghoul

Ghouls or necrotic post-humans are decrepit, ragged, almost rotting, zombie-like mutants, victims of massive radiation poisoning.

Origins
Limited exposure to radiation for a longer period of time sometimes causes humans to transform into ghouls, although rare cases of faster transformations into ghouls are also not unheard of. Exposure to radiation typically result in sickness followed by death and the x-factor that will lead to mutation upon exposure in lieu of the typical outcome remains unknown.

Most known ghouls in the Core Region were created from vault dwellers living in Vault 12 under the city of Bakersfield (better known as the Necropolis after the Great War). As part of the vast Vault Experiment Program, the Vault 12 door was designed not to close properly. Thus, massive amounts of radiation attacked those within the Vault, most of whom either died or mutated, eventually turning into ghouls.

Ghouls from other regions most likely originate from other, similarly ill-fated shelters that shielded their residents from radiation enough for it not to kill them, but enough for them to mutate.

In the Capital Wasteland, many ghouls currently alive were born long after the War and succumbed to radiation poisoning much later, due to high levels of radiation in Washington, DC and its surroundings.

Biology
All ghouls are completely sterile. There is only one generation of ghouls in the wasteland and it is the last. Ghouls created in the Great War of 2077 were still alive during Fallout (2161), Fallout 2 (2241), and even Fallout 3 (2277). Some ghouls live longer than normal humans, while the majority do not. It has to do with cellular level, and the ability for some ghoul DNA to make more copies of themselves. Basically, cells only contain enough material to be duplicated X number of times. In a ghoul, sometimes, additional genetic material is added. The unnaturally long lifespan is also due to a mutation within the autonomic nervous system of certain individuals following exposure to specific combinations of ionizing radiation with wavelengths below 10 picometers. This mutation disrupts the normal process of decay in the neurotransmitters along the spinal cord.

Specifically, the transmitters affected are those responsible for cardiac and respiratory function in a healthy human. These transmitters are continually regenerated after mutation, carrying sufficient oxygen to sustain the life of the subject while being insufficient to retain skin elasticity and avoid necrosis, the result of which is the corpse-like appearance of post-mutation humans.

Ghouls are generally as intelligent as normal humans. However, their repulsiveness makes the life of a ghoul difficult at best - only the most tolerant human communities accept them as anything more than monsters. Some ghouls eventually go mad and are called "feral ghouls", giving all ghouls a bad reputation. These feral ghouls create the misconception amongst regular people that they are Zombies, only alienating Ghouls further.

Ghouls are, naturally, immune to radiation. Radiation poisoning can't really get worse for the ghouls. In fact, many ghouls now thrive in radiation, making their homes near locations with acute background radiation. The "Glowing Ones" actually enjoy large amounts of radiation, which they describe as being "comfortably warm".

the guard outside of underworld thinks that super mutants dont bug them because "what ever turns humans into them dosent work on us" this likely means that ghouls are imune to the FEV

Feral ghouls
Feral ghouls are those of the unfortunate mutants that lost their minds and became aggressive. Their intelligence is almost completely gone and they react mostly on instinct, making them basically Zombies. They are, however, friendly with normal ghouls.

Glowing ones
Some ghouls absorbed so much radiation that they glow in the dark. They are called the Glowing Ones and are often considered outsiders even by other ghouls. The Glowing Ones also emit radiation, so they can be dangerous to normal humans. This variation also takes much more punishment than their feral counterparts.

Born ghouls
Through cruel experimentation on humans by Dr. Sebastian at the Reservation, Born Ghouls were created. They are ghouls who were not mutated smoothskins, but who were actually born into ghoul-dom.

Appearances in games
Ghouls have appeared in every Fallout game.

Trivia
A ghoul is described as "Bloodman" in an old Fallout concept art.

Quotes

 * There ain't any ghouls but old ghouls. We're all sterile, see, but we're incredibly long-lived. We're the first and last generation of ghouls. - Typhon
 * Without medical technology, all of us Ghouls are going to die off in the next 20 years, anyway. - Gordon (Gordon dialogue file)
 * [Say, how do you make a ghoul?] With silver-bells and cockleshells and… Boy, you are dumb, aren’t you? Severe radiation. That’s how. How do you think? You know, many bombs go boom, flash of light and heat, flesh burns off, but you don’t-quite-die-type severe radiation? - Wooz (Wooz dialogue file)

Controversy
There is some controversy even among the makers of Fallout games about the origins of ghouls. While Tim Cain said explicitly that ghouls are only a result of radiation, consistently with the "SCIENCE!" of the 1950s, Chris Taylor said that a mix of both radiation and FEV was in play (which would probably mean that there aren't any ghouls outside the Core Region). While Chris Avellone initially supported the latter view in his Fallout Bible, he was later convinced to support the radiation-only version , which is also confirmed in Fallout 3.