D, The white blood cells of sick people may not be able to efficiently handle the weakened or dead form of the pathogen.
Explanation:
Vaccinations are essentially just weak diseases we use to create antibodies so the body can protect itself from the full disease. If it sees it once, it can defend itself, is the theory. This is choice D.
Why not A: The weakened form will be attacked by the immune system, but maybe not enough, because the person is so sick that the immune system is busy elsewhere.
Why not B: The medication is not what combines with the vaccine to cause problems. It is the body being overworked.
Why not C: The digestive system isn't the issue. You don't eat diseases.
