Antibiotics act by blocking certain biochemical pathways which are essential for the survival of bacteria like cell wall formation, protein synthesis etc. For example: Penicillin is an antibiotic. It blocks the formation of cell wall in bacteria, which makes bacteria weak and ultimately results in death of bacteria. However, antibiotics cannot be used against viruses. This is because, viruses do not possess any cellular machinery of their own to function. In fact, they get integrated with the host cell and use machinery of the host to survive. They are non-living outside the host and they have very few biochemical pathways of their own. Hence, it is difficult to find virus-specific targets.