 In both cases, the likelihood of mutations is exacerbated by the speed with which viruses reproduce, which provides more opportunities for mutations to occur in successive replications.Billions of viruses are produced every day during the course of an infection, with each replication giving another chance for mutations that encode for resistance to occur.Multiple strains of one virus can be present in the body at one time, and some of these strains may contain mutations that cause antiviral resistance.