Cold sores are always highly contagious and the cold stores stay contagious for as long as you have the cold sore.
Even when the cold sore is scabbed over the cold sore is still contagious due to the virus still being around.
You can kill off a cold sore by applying apple cider vinegar or even baking soda paste to the cold sore.
Apply the apple cider vinegar to the cold sore 3 to 4 times per day and the cold sore will usually go away within 4 to 5 days as long as you continue to use the apple cider vinegar on the cold sore everyday.
Avoid kissing someone else or don't let them kiss you either when you have a cold sore because they can spread and are contagious.