What causes cold sores?

Kissing, sharing utensils, touching a cold sore with your finger, and other ways of coming into close contact with someone who has a cold sore can increase your risk of developing one, as HSV is contagious. Once you have an HSV infection, you are more prone to getting cold sores. Various factors can trigger the appearance of an breakout, such as stress, illness, hormonal changes, or skin injuries.