Acyclovir is an antiviral medication that scientists developed in the twentieth century to treat herpes, a disease caused by the viruses herpes simplex virus-1 and herpes simplex virus-2, or HSV-1 and HSV-2. The viruses cause painful lesions to develop on the mouth and genitals. Herpes is a very common condition and acyclovir was one of the first medications to effectively heal and reduce the duration of lesions caused by HSV-1 and HSV-2. Scientists at Wellcome Research Laboratories in Research Triangle Park, North Carolina, developed acyclovir with the specific aim of inhibiting the replication process of HSV-1 and HSV-2 in a host’s cell, which was a novel approach to drug development at the time. Physicians have since used acyclovir to treat other diseases caused by herpesviruses. Scientists have developed new drugs for the treatment of genital herpes based on acyclovir’s mechanism of action and, as of 2024, physicians still prescribe acyclovir to treat oral and genital herpes, providing relief to the millions of people living with herpes globally.