The spread of the modern-day cold sore has been traced back to the Bronze Age and linked to the advent of kissing in a new DNA study.
Research suggests the HSV-1 strain of the herpes virus arose in the wake of vast migrations of people from Eurasia to Europe around 5,000 years ago.
The migration led to both denser populations, which drove up rates of transmission, and new cultural practices being imported from the east, including kissing, according to the study.
Scientists at the University of Cambridge leading the research have become the first to uncover and sequence ancient genomes of the virus, which currently infects some 3.7 billion people worldwide.
Previously, genetic data for herpes only went back to 1925, but the team hunted down four samples from human remains dating over a 1,000-year period.
By comparing older samples with ones from the 20th century, they were able to develop estimates for a timeline of evolution.
Facial herpes is spread orally, and the researchers point out the earliest known record of kissing comes from a Bronze Age manuscript from South Asia.
The custom may have spread westward along with migration and coincided with the spread of HSV-1, according to the study.
Centuries later, the Roman Emperor Tiberius would try to ban kissing at official functions to prevent disease spread – a decree that researchers believe may have been herpes-related.
However, for most of human prehistory, the strain would have been passed down "vertically" – from infected mother to newborn child.
Co-senior author Dr Christiana Scheib, research fellow at St John's College, Cambridge, and head of the ancient DNA lab at Tartu University, said: "Every primate species has a form of herpes, so we assume it has been with us since our own species left Africa."
"However, something happened around 5,000 years ago that allowed one strain of herpes to overtake all others, possibly an increase in transmissions, which could have been linked to kissing."
"The world has watched Covid-19 mutate at a rapid rate over weeks and months. A virus like herpes evolves on a far grander timescale," said co-senior author Dr Charlotte Houldcroft, from Cambridge’s Department of Genetics.
"Facial herpes hides in its host for life and only transmits through oral contact, so mutations occur slowly over centuries and millennia.
"We need to do deep time investigations to understand how DNA viruses like this evolve."
Two thirds of the global population under the age of 50 now carry HSV-1, according to the World Health Organization.
Cold sores are mostly a source of discomfort for those suffering from them, but coupled with other health complications such as sepsis or even Covid, the virus can be deadly.
Researchers say only DNA from hundreds or thousands of years ago can help to explain how viruses such as herpes and monkeypox evolve in tandem with human immune systems.
They took the samples used in the study by extracting viral DNA from the roots of the teeth of four individuals.
Herpes often flares up with mouth infections, and at least two of the individuals had suffered gum disease, a third of whom smoked tobacco.
The oldest sample came from an adult male excavated in Russia's Ural Mountain region, dating from the late Iron Age around 1,500 years ago.
The others were a female excavated from an early Anglo-Saxon cemetery in Cambridge, a young adult male from the late 14th century also in Cambridge, and another young adult male likely from 17th century Holland.
"Only genetic samples that are hundreds or even thousands of years old will allow us to understand how DNA viruses such as herpes and monkeypox, as well as our own immune systems, are adapting in response to each other," said co-lead author Dr Meriam Guellil, from Tartu University's Institute of Genomics.