Quarantine worlds are marked as forbidden for visitation, settlement, or contact of any kind. Some might be seemingly benign verdant spheres, others might be foreboding rocks, but the Imperium has determined there is some terrible threat that outweighs any possible usefulness they might contain. Such threats range from biological agents to xenos species infiltration, and even to possible Warp-taint and other unholy threats. Whatever the catastrophe, potential or historical, it is viewed as a far greater risk than any reward it could bring.