Question

Why is a particular medicine is effective against a disease caused by a particular group of orgaanisms and not by others?

Answer 1

he medicine is prepared considering the pathway followed by the particular group of microorganisms. For example, Bacteria make cell wall. The antibiotics block the process of building the cell wall in bacteria. This kills the bacteria.
But viruses do not follow this pathway, so their growth cannot be control by using antibiotics. Thus a particular medicine is effective against a disease caused by a particular group of organisms and not by the others.