Antibiotics can fight only with bacterial infections and do not work against viral infections, like flu, common cold etc. Because Antibiotics commonly block biochemical pathways, important for bacteria. These antibiotics inhibit the growth of bacteria and kill them. But the same antibiotics do not work on viruses. Because viruses do not use biochemical pathways such as bacteria and they have few biochemical mechanisms of their own. Viruses use the host's cells to replicate. Finding the targets for the drug is very difficult. That may harm the cells of the host organism. Variation of virus is also another reason. That is why it is difficult to make antiviral drugs.