Not all Pre–Corporation Rim sites are alien-contaminated, obviously. A lot of them became long-term occupation sites, the nucleus of what are now independent polities or Corporation Rim–owned planets (page 57, paragraph 6, Introduction to the History of Human Expansion, Volume I, eds. Bartheme, de la Vega, Shanmugam, et al, Cloud Sun Harbor University Press). But abandoned sites are sometimes abandoned for a good reason, and humans didn’t understand back then how careful they needed to be with alien remnants. (Or maybe they did and they just didn’t care. I mean, let’s be honest, which one is more likely? I’m just making an observation here.)