The HSV brings along its own thymidine kinase to replicate itself on salvaged pathways of DNA synthesis. Thymidine kinase is encoded by the virus as well as our own bodies. Drugs can be created to target the thymidine kinase of the virus. Without the extra hydroxyl group, chaining cannot continue. HSV is very effective at adding phosphates with its own thymidine kinase but the chain stops because of lack of the hydroxyl group.
Healthy cells can ignore acyclovir, but infected cells activate acyclovir.
Pathways are worked on by enzyme catalysts with great specificity. Acyclovir relies on gene that is created by the virus. HIV is lymphocyte specific and targets CD4 cells. The env gene binds to CD4 cells gaining entry to cells. The virus gets into the cell, fuses with the cell and releases genome into the cell.