Nobody really knows the origins of RAD-X, but it is known that it affects the beings it infects. RAD-X stands for Radiation Accelerated Disease, X being the strain of the virus and placeholder variable. It's known for attacking mostly the brain of its host, but may also cause deformities (extra limbs, different skin color, etc).
The disease has multiple stages, here's the order it usually goes.
RAD-1 symptoms (stage 1) -> RAD-2 symptoms (stage 2)* -> RAD-3 symptoms (stage 3)* -> RAD-4 symptoms (stage 4)* -> RAD-5 symptoms (stage 5)* -> RAD-6,7 or 9 symptoms (final stage)
*If a person contracts RAD-2, they won't go pass the 2nd stage, just like a person with RAD-1 won't go past the 1st stage, and so on.
10 strains of the virus are known as of now. The higher the "number", the more the disease affects its host but the less contagious it gets. Victims are beyond a full recovery after stage 4-5 due to brain damage.
RAD-1: Flu-like symptoms, nothing special.
RAD-2: Signs of "mutation" begin to appear. Slight change in skin pigmentation to pale green/yellow (body parts, not the entire skin), hair loss, etc.
RAD-3: Same as rad-2, except symptoms are more intense. Loss of cognitive abilities and coordination begins. Any mutants in this stage or below are called “Refugees” by other survivors, as some still believe in a cure.
RAD-4: Marauders. Rotten, deformed faces and bodies. The intelligence loss is noticeable as the host is incapable of doing complex tasks like numerical operations and properly typing on a keyboard, for example. Marauders are natural leaders for the mutated.
RAD-5: Sapiens. Look like marauders except no intelligence at all - they are incapable of operating firearms or speaking, even if barely. To quote ATF 1's guide: "Mindless mutants, at this point they retain no Human attributes with the exception of their body structure. Well, most of it anyway."
RAD-6: Hitmen. Uncontrolled growth of certain muscles and bone structures begin to make those mutants less human-like. "Usually extremely strong in only one limb, this limb can knock a fatal blow."
RAD-7: Crushers. Tumours cover the body of this deformed, extremely muscular and agile creature. "Usually extremely strong in both limbs, they are extremely fast for their size and can deal fatal blows."
RAD-8: Creeper or Leacher. Extra limbs are extremely common among this kind of infected. Shoots infected cells from distances of up to 5 meters or 15 feet. "The least Human-like mutant. They are small in size, with 2 heads and 3 arms."
RAD-9 (ONLY ON ATF 2): Called Malevolent and Malicious Ones, not fruit of 'naturally' evolved RAD-X but instead the result of an engineered strain of the virus. "These mutants are beyond all hope and must be killed. They are not even allied with the virus-human hybrid mutants."
RAD-10 (ONLY ON ATF:DW): Carriers (Carrier Crusher, Carrier Marauder, etc). Also an engineered strain of the virus. RAD-10 may give humans symptoms similar to RAD 1-8, and may also affect plants and fungi. Plants appear to have stronger fibers with a flesh-like texture on the outside and minor movement. Some even develop "spore sacks", which are highly infectious. A few survivors also report seeing creatures which combine both - a human covered in spore sacks, for example.-------------
Destroying the virus and or virus carrier is usually the only way to destroy the virus. Primitive homemade vaccines have been shown to have various effectiveness, except that they only work on RAD-1 and rarely RAD-2 infected.