Many ST champions can take Frost damage but can't be Chilled or Frozen. It's a similar situation. Constructs/Undead are naturally resilient to Diseases and Poisons, but they aren't immune to the initial chemical reactions of the impact. A corrosive poison is going to leave a pockmark when it hits the side of a Construct's arm, but it's not going to do a significant amount of damage. Similarly, a disease that doesn't rely entirely on biological compounds is going to do minor damage before it fizzles out due to a lack of resources for spreading. Being Diseased or Poisoned means that you have a systemic affliction that continues to harm you as your body fights it off. Constructs/Undead don't need to do this because their bodies don't facilitate their spreading. Also, see War of the Worlds.