“STD transmission?”

I have been dating a girl for about 4 months now and just the other day had her first herpes outbreak. I have never had an outbreak before and as far am aware don't have it. I am just curious about how the virus is transmitted, aside from sex obviously, like how she would contract it if I didn't have it and none of my previous partners ever showed signs of it. Could I have had it for years without an outbreak or could she have contracted it before we started dating and is just now having an outbreak?