The Blob, or XE037 as the XEDRA scientists called it, is one of the three big factions in Cataclysm: DDA (originally, it was the only faction, AFAIK).
It is the source of almost everything: The Zombies, The Mutant Animals, The Mutant Creatures, what Mutagen is made out of, what Blobs are made out of, and most importantly is what has infected every living being on the planet.
Every living thing has some Blob in it. Every human, every animal, every mutant. Different lifeforms have different reactions to the blob.
Mammals: In mammalian bodies, the blob will multiply until an amount equaling 1% is inside the body. Then it becomes inert. However, upon the death of a mammal weighing at least 50 pounds, The Blob will become active and reanimate the corpse, with the features of the reanimated body being based on cause of death, damage done to the body, foreign bodies inside the corpse (think all of the cybernetic zombies), what resources are available to the blob and what it believes are currently the most desirable traits (it’ll either make a zombie that’s physically strong/has some kind of power, or make a zombie that’s smart, like a Zombie Master, Zombie Scientist, or Zombie Grenadier). Zombie Animals seem to be exclusively physically based, though, with powers being unheard of (Smoky Bear notwithstanding).
Arthropods, Fish and Annelids seem to grow in size significantly. This seems to be limited, because only so many animals grow in size (you don’t see thousands of giant spiders, for example). Entire ant colonies seem to mutate, though, so I’m guessing that the blob will choose to concentrate in certain animals or groups of them to induce mutagenesis. Oddly enough, some fish turn into zombies.
Birds seem to be unaffected. For now, at least…
The blob also has mutagenic properties in its purest state, which is why Tainted Meat and Blob Globs can be used to make Mutagen. The Blob is essentially sentient stem cells, and these make up all of the Subprime/Nether lifeforms, with the exceptions of Triffids and Fungals. Adding different materials and DNA to mutagen allows it to create different mutation paths. When they mixed it with animal DNA, they got Bird, Lizard, the Beasts, etc. Plants got the Plant Mutation tree. Mixing it with Pharmaceuticals got Medical. Focusing it and giving it via injection got serums, and they also derived Purifier from it. Alpha was made with the lofty goal of making the perfect human, before everything went to Hell.
Chimera was made as a sort of “crowd control” mechanism; the unstable mutant would kill as many zombies as possible before the numerous unstable mutations killed it, Elfa and Raptor were never able to be tested, because of the fact the world had ended by that point.
Anyway, the XEDRA scientists were actually being pretty responsible about the whole thing, except they didn’t know that The Blob was sentient, and it caused a sort of spatial surge at multiple laboratories at once and broke out in the carnage, infected the water table, then the rest of the population. People didn’t realize what was happening at first (in the States at least) because of riots over a peacetime draft. However, that ended quickly (5 days between the first zombie reanimating and the start of the game).
Anything else you want to know?