M.P.
I used to often get cold sores. They are caused by a herpes virus. One cause for having them is having had chicken pox, tho you can be first infected in other ways. I had chicken pox. Many years ago a friend wouldn't even let me hold her baby because I did get cold sores from time to time, even if I didn't have a cold sore at the time. Her sister was a medical intern and said that they are most contagious before they break out.
I asked my doctor and he said it was safe to hold the baby but to be on the safe side not to kiss her on her face and hands. He said babies do have a high resistance against catching cold sores and he wouldn't be concerned if this were his babies but but agreed that I needed to honor the mother's boundaries.
I have kissed many adult lips while having a cold sore and no one has gotten a cold sore. I don't kiss babies or children when I have a cold sore just to be on the safe side. Most mothers request this anyway. I stopped being concerned about cold sores or their being contagious many years ago after doing research and reading. I suggest you google cold sores and read up on them