reason. They are infected my a man made disease. This page explains their history and what they look like.
In 2104 the scientist company known as New World developed an injection that made the subject perfect in every way.... They're first test subject was injected with the first product he grew spiky hair and went insane. He (known as Subject Alpha) was put in a containment cell for monitering. After a year of keeping an eye on him, he eventually sprouted horns and his eyes turned red. Now known as the Demon, he broke out off the cell in 2106 and masacared the scientists. A bootle of the infection spilled into a fleeing scientists mouth, and he too grew spiky hair and went insane. The Demon realized this and infected all the surviver scientists. Raiders were constantly raiding towns and other areas for more soldiers. They soon attacked the Megaton Nuclear Power plant and managed to blow it up. The huge amount of nuclear bombs and reactors in the plant caused the entire planet to be wiped out by radiation. Or so they thought..... Raiders have been trying to Wipe out all life in the Wastes ever since.
Raidrs can change form depending how long they have been infected. When they get to two months infected, the infection has seeped deep into the hosts heart, so they can no longer be cured. Trying to cure them after this stage will kill them. After six months infected, they start to mutate. Their infection has started to take hold of the body and is reproducing slimy slugs containing more infection. The tenticales from the slugs can be seen sticking out of the Raider after this stage. They eventually come out of the Raiders mouth or wounds etc. and the Raider will continue as normal. After around a year of being infected the infection reproduces again but the offspring is much more deadly. They kick out the origional infection and take over. This new infection is known as the work of the devil. They grow horns and their eyes bgin to glow red. They are five times as dangourus as regular Raiders. They have a needle which comes out of under their wrists. Injecting another uninfected person will suck their blood out of them, so the Demon looks more like a regular person, but still as dangerous.