At what stage do cold sores become contagious?

Herpes. Asymptomatic shedding is very real and one of the biggest reasons why it's possible for people with herpes to pass herpes virus on to a sexual partner without having any signs or symptoms. That's pretty much what asymptomatic means: you can't see or feel that the virus is present and active on the surface of the skin. Thus, the person with herpes is always contagious.