Hi,
A cold sore will cross the border between the lip and the skin. Once you get them you cannot prevent them. That's why anyone with a cold sore should not get their sore near another person. Once you get the sore you can use antivirals like Valtrex or zovirax ointment. At 4 months of age, you can use the zovirax ointment or Neosporin.
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Dr. Bornstein