Cold sores are caused by infection with the herpes simplex virus, most commonly herpes simplex virus type 1 (HSV-1), though herpes simplex virus type 2 (HSV-2) can also cause them less commonly
. People usually become infected with HSV-1 in childhood or young adulthood through close skin-to-skin contact such as kissing or sharing utensils, towels, or other personal items with someone who has an active cold sore
. Once infected, the virus remains in the body for life, residing dormant in nerve cells. Cold sores occur when the virus reactivates and travels to the skin around the mouth or lips, causing painful blisters
. Reactivation can be triggered by factors such as stress, illness (like a cold or fever), sun exposure, hormonal changes (e.g., menstruation), fatigue, skin trauma, or a weakened immune system
. In summary, cold sores come from the herpes simplex virus, primarily HSV-1, which is acquired through close contact with an infected person and can reactivate later due to various triggers to cause outbreaks of sores around the mouth
