A cold sore is a blister or a cluster of blisters that typically form on or around the lips and mouth. Affected people can experience recurrent bouts of blister outbreaks, often triggered by sun exposure, hormonal fluctuations, stress, illness, or injury. The condition is caused by an infection by the herpes simplex virus. There are two types of the herpes simplex virus, herpes simplex virus type 1 (HSV-1) and herpes simplex virus type 2 (HSV-2), both of which can cause blisters and sores on any parts of the body. Classically, HSV-1 is thought to be the main culprit for causing cold sores. Infection by the virus is extremely common as the virus is highly contagious. The virus can spread through saliva, skin to skin contact, or by touching a contaminated surface. While there is no cure, topical and oral treatments are available to speed healing and also to prevent or control outbreaks.