Why is it difficult to cure viral infections?
Because viruses can change their shape and composition in fraction of seconds
Viruses are said to be on the borderline of living and non-living. They do not exhibit any of the vital life processes like respiration, excretion etc. and do not have the necessary cellular machinery for any of them. For reproduction, they use the host's system and resources. Therefore it is difficult to attack them with drugs. Also, viruses change their shape and composition in fractions of a second.