Stress is one of the best-known environmental triggers of cold sores, and studies and anecdotal evidence support the positive effect of keeping stress in check on flare-ups. Excessive sun exposure can also prompt cold sores, which is yet another reason to use sunscreen. A lowered immune system can also cause them, as can skin-to-skin contact with a person currently in the grip of a flare-up. The risk of catching the virus from another person is highest when they have blisters that are oozing, although they can spread the virus even when blisters aren't visible.
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