At the end, infected colonies are just humans or others aliens being aligned and mutetaed with an by chaotic entities, so, it could be that they are in fallen human colonies or other places across the siruis sector.
Sounds plausible enough. Mostly I just wanted to give an example.
But from what I read, they didn't seem like they were mutated by Chaos stuff like the Havoc Demons, more like a specific parasite infected them.
