Visceroids are the results of uncontrolled, irregular Tiberium mutation.
Visceroids were disturbing creatures that sometimes form when organic life dies to tiberium exposure. As flesh breaks down after death, it has the potential to mutate into a visceroid, a crude Tiberium-based lifeform. Visceroids often appeared to be blob-like masses of flesh that sometimes had a few recognizable features from those they once were left over within their structure, and were highly erratic in nature. Some types were even capable of combining their biomass to essentially make a larger visceroid.
The Visceroids of 2030, during the Second Tiberium War, can be effectively described as a second generation of the creature. They can be defined as a collection of pre-sentient protoplasm, with lightning-quick, extremely strong pseudopods and the ability to absorb great amounts of kinetic force. Smaller entities were relatively docile (colloquially referred to as "baby" visceroids), but larger specimens were capable of destroying entire cities, or wreaking havoc even in military bases, due to their speed, ferocity, and resilience.
Second generation visceroids were most notable for their ability to combine. Two "baby" visceroids could form into an "adult" visceroid, which was essentially just a larger version of the former creature, being stronger, more durable, and more aggressive as a result. While babies would mostly run from potential enemies, and defend themselves when necessary, adults would actively and aggressively attack any potential threat near them. Even GDI's large and powerful Titan walkers were vulnerable to attack by aggressive adults.
Additionally, all Visceroid forms thrived in the presence of Tiberium. Simply being in a Tiberium field or being exposed to the substance was enough to allow them to regenerate. As a result, Visceroids were often found near Tiberium, and were always associated with it. Tiberium was required for their sustenence, and also required for them to even initially form. Second Tiberium War visceroids were so strongly associated with the substance that simply dying caused Tiberium crystals to begin to sprout from the site of their death.
The Visceroid population is directly tied to levels of Tiberium poisoning. Tiberium gas is a corrupting agent, breaking down virtually any living tissue it comes into contact with. Long term exposure cannot be treated and causes a spontaneous breakdown of the affected tissues nearly instantly, as Tiberium saturation within them reaches a certain point. Smaller Visceroids can merge to form adult forms, which are significantly more dangerous.
Unsubstantiated reports exist that some visceroids can reproduce, but most are unfortunate humans transformed by Tiberium.
In Tiberian Sun, baby visceroids are created when human units (military and civilian alike) lose the last portion of their health by Tiberium poisoning, which is triggered by walking over Tiberium fields, or come in contact with Tiberium waste clouds from veinhole or chemical missile. Also, one viseroid will spawn if a larger building (note: automated defense turrets and silos are exceptions) is destroyed by Tiberium cloud corrosion. Baby visceroids are docile and weak, however when two baby visceroids encounter they will merge into one adult visceroid, which is viciously aggressive, deals serious 'melee' damage to infantry, armor and structures alike, uncontrollable and will attack randomly without distinction; since player's force is not ordered to attack visceroids on sight, those creatures may slip in and destroy the player's base if left unnoticed.
In one of the last Nod missions (Establish Nod Presense or Protect Waste Convoy), the player will be instructed by CABAL to 'infect' enemy base via Chemical Missile. However in practice such decision must be made cautiously due to the uncontrollable nature of visceroids; sometimes they just do not harass GDI as planned, instead can be quite a nuisance for conventional NOD siege force, as NOD commander may be forced to deal with adult visceroid threat before engaging the remnant of GDI, causing unnecessary delay and casualties. Contrary to popular belief, launching chemical missile directly at large infantry concentration is not the best way to create visceroid since the impact damage from the the missile will kill soldiers instantly rather than having them transformed.