10 Causes and Treatments for Cold Sores

Cause: Illness

One potential trigger for those carrying the cold sore virus lies in the name, getting a cold.

For some, getting a cold or the flu can cause an active lesion to appear, especially if a person has a fever (yes, as in fever blister) [source: Mayo Clinic].

The connection has to do with the immune system, which helps keep HSV-1 in check. People with weakened immune systems find themselves fighting off cold sore outbreaks more often than those who aren't sick [source: WebMD].