The easiest one to understand was radioactivity. Radiation can penetrate objects. Only the thickest of lead shielding could stop it. Maybe there would be some other form of radiation that not even lead could stop. Although reseach into atomic physics would discover other realities perhaps even more threatening to our boundaries than radiation, for the time being, its contamination was what we could appreciate.
Contamination is an assault on purity.
Something has come into where it doesn t belong. A boundary violation. The next strong example of this theme was in ecology. The condition of the planet was becoming fouled. Whereas we used to think that we could bury our garbage we realized that wherever we buried it it was still with us on planet earth.