Question

how antibiotics act on bacteria why it is difficult to prepare antiviral medicines

Answer 1

Antibiotics enter in the body and stop/kill the growth of the microorganisms .
Designing safe antiviral medicine is difficult due to the viruses use the host to replicate . This makes it difficult to find targets for the drug that would interfere with the virus without also harming the host organism's cells.

Answer 2

Antibiotics block the biochemical path of bacteria and this is the way which antibiotics work.

virus have different biochemical path so it is difficult for us to know how they affect us and so it is difficult to make.