Cold sores, also known as herpes labialis, orolabial herpes, or fever blisters, are contagious sores on the lips and mouth caused by the herpes simplex virus (HSV). Infections typically occur after direct contact with a sore, although transmission can occasionally occur when symptoms aren’t present. After an initial episode, the virus remains dormant in sensory nerves, and about one-third of people go on to experience recurrent episodes, which can be caused by factors like stress, hormonal changes, fever, UV light, and certain medications. Cold sores are distinct from canker sores, which are not caused by HSV and are not contagious.