I suppose most people carry the cold sore virus in any case.

I'm not sure what the incidence is, but it's far short of everybody.

The cold sore virus was used for one of the oddest bits of evolutionary detective work I know of. It's a descendant of the genital herpes virus. By counting the variances between the two in DNA base pairs and assuming the usual rate of genetic drift, we can infer that the split took place about three million years ago.

Now, the mode of transmission of genital herpes is a pretty inescapable part of continuing the human race. But catching oral herpes isn't. Conclusion: kissing evolved about three million years ago.

Or perhaps it was harmonica playing.