Why Is It Difficult to Develop Drugs That Specifically Target

Why is it difficult to develop drugs that specifically target viruses and eukaryotic pathogens?

A) The drugs often inflict collateral damage on our own cells.
B) The ethical concerns with testing these drugs are too great.
C) Viruses and eukaryotes evolve too rapidly for drugs to be effective.
D) Security enzymes break down these foreign drugs before they can exert their effects.
E) There are often duplicate metabolic pathways that pick up the slack from any disturbance caused by these drugs.