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 the 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, such as vomiting/nausea, dizziness, fatigue, light-headed, fevers/chills, painful spasms in the muscles, loss of appetite, swelling (in minor areas such as around the fingers, eyes, cheeks, and stomach.), phlegm-filled cough, hyper-active sweating, running nose, chest pressure, and anything revolving around basic flu symptoms. Often times infected will not experience all the symptoms listed above, and depending on the area of infection is where the symptoms may vary side effects wise.
RAD-2: Signs of "mutation" begin to appear. A slight change in skin pigmentation to pale green/yellow (body parts, not the entire skin). Depending on where the infection first started is where the mutation of skin will be the worst (for example, if the host was bitten on the shoulder, it would take a lot longer for the skin tone deforming to reach the feet then neck.) other side effects include, lost of hair in large to small amounts (depending on bite area), increased dizziness and restlessness, increased painful spasms in muscles, fevers, vomiting/nausea, severe dehydration. etc.
(See RAD-1 for rest of symptoms)
RAD-3: The Same as RAD-2, except symptoms are more intense (See RAD-1 and RAD-2). 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-3's are typically found mindlessly wandering.
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 for 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: Undead. They are Sapiens but their limbs begin to snap and contort, they are extremely weak, and do not do anything unless ordered to by a Marauder.
RAD-7B: 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-8B: 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-7A: Creeper. One of the many transformations a Sapien/Undead can undergo. They are agile and stealthy, perfect for suprise or night attacks. They still retain their former Sapien/Undead bodystructure unless they move ahead in mutation.
RAD-8A: Leecher. 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 a 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."
(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.
(RAD-10 HAVE BEEN REMOVED FROM DEEP WINTER! DO NOT RP AS ONE, OR YOU WILL BE CONSIDERED AS AN FRP! ALSO, THIS DOES NOT INCLUDE PEOPLE THAT CARRY THE VIRUS! BECAUSE, THESES KINDS OF CARRIERS ARE NOT MUTANTS; AND THEY LOOK NORMAL.)
-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.