How does acyclovir prevent viral replication?
Acyclovir is used to treat Herpes virus infections (that is kind of inaccurate - you can't get rid of the Herpes virus, so the drug is used to treat the flare-ups).
The virus has a kinase enzyme that phosphorylates the base Thymine (T), so in cells that have active virus, Acyclovir gets phosphorylated. The phosphorylated form of acyclovir then inhibits DNA replication in viral infected cells, while leaving other cells (non-infected) alone (since it is not phosphorylated by the viral thymadine kinase).
In this way, viral DNA is not able to replicate in the presence of Acyclovir.