Never for sure: There is no way to tell for sure. Some people shed the virus all the time, even when they do not have a cold sore, while cold sores are most contagious when they are oozing fluid, the virus can be transmitted to others even during times when you have no blisters.
Answered 11/8/2014
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4 doctors weighed in across 2 answers
A doctor has provided 1 answer
4 doctors weighed in across 2 answers
A doctor has provided 1 answer
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