This is because anti bacterial medicine block the formation of cell wall of bacteria. When bacteria don't have cell wall they don't divide hence they die. While viruses don't have their machinery of own, when they enter into our body they use our cells as machinery. So to kill virus we have to kill our cells as well. Hence making anti viral drugs is more difficult than making anti bacterial drug.
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This is because virus is intermediate organism b/w living and non-living.They enter living beings body and becomes living,but when outside living beings body it becomes non-living,when it enters living body it uses that host machinery to sustain life and thus if anti virus drugs are made they will kill the host instead of killing virus because of same machinery to sustain life.