This is because anti bacterial medicines block the formation of cell wall of the bacteria. When, bacterias dont have cell wall, they dont divide and hence they die. While, viruses dont have their machinery of own. When they enter into our body, they use our cells as their machinery. So, to kill viruses we've to kill some of our cells as well. Hence, making anti-viral drugs is more difficult than making anti-bacterial drugs
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It's is difficult to make anti viral medicines rather than antibacterial bcuz the bacteria can be killed easily by the antibacterial medicines by destroying their cell wall but viruses enter our cell and leave there DNA in our cell and then reproduce this antiviral medicine will kill our whole cell while killing the virus.