Herpes viruses have been infecting and co-diverging with their vertebrate hosts for hundreds of millions of years. The primate simplex viruses exemplify this pattern of virus-host co-divergence, at a minimum, as far back as the most recent common ancestor of New World monkeys, Old World monkeys, and apes.

Humans are the only primate species known to be infected with two distinct herpes simplex viruses: HSV-1 and HSV-2. Human herpes simplex viruses are ubiquitous, with over two-thirds of the human population infected by at least one virus.

Here, we investigated whether the additional human simplex virus is the result of ancient viral lineage duplication or cross-species transmission.