Answer
We could tell Cathy that viruses do really mutate and have been doing so since they first appeared. Acyclovir probably did not work for her because the virus likely mutated to use a different mechanism for replication. We could advise her to try the new medication.
Work Step by Step
Drugs like acyclovir work by inhibiting specific steps in the replication cycle of the virus. If a virus has a mutation that allows it to replicate without the step inhibited by acyclovir or if the virus can replicate by using another step, then the drug will not work.
