Question

Why are antibiotics effective against bacteria but not against virus?

Answer 1

Bacteria have cell wall. Antibiotics block the formation of cell wall in bacteria which make them weak. therefore, bacteria die. Virus don't have cell walls. That's why antibiotics are not effective against them. 

Answer 2

antibiotucs are effective against bacteria but not against virus because they perform their activies with the help of their own host cells while bacteria has a different mechanism.
antibiotics act by a blocking the biochemical pathways which are essential for the survival of the bacteria.viruses so not use this mechanism.