It is most difficult to prepare drugs against

Viruses do not have defined body structures or body physiologies. They use the machinery of host cells and live life on that. Hence, it is difficult to control them because we cannot prepare drugs to arrest any step of their physiology because those drugs will actually affect our own (host) cells and harm us. Hence, it is very difficult to prepare drugs against viruses. On the contrary, the physiologies of bacteria, fungi and protozoa are well known and the reactions are different from host cells. We can easily prepare drugs to attack those specific reactions.