Dear Student
Antiviral drugs is difficult in making as compared to antibacterial medicines because:
1. As viruses can only grow, divide and live inside the body of a host they do not possess any cellular machinery of their own to function. Therefore, they have very few biochemical pathways of their own. Hence, it is very difficult to find virus-specific targets.
2. Viruses can mutate continuously over a short span of time, so the medicine which act once successfully may not be very much effective next time.
Regards