Cold sores are caused by a virus, the herpes simplex virus type 1 (HSV-1). There are two types of the herpes simplex virus: HSV-1 and HSV-2. The former is responsible for herpes labialis, a herpes located around the mouth, most commonly on the lips. This type of herpes is the one commonly referred to as cold sores or fever blisters, while HSV-2 causes genital herpes. Generally speaking, herpes is a rash of the skin and mucous and is characterized by reddening of the affected area followed by blister formation, which can burst and thus lead to scabs.
The primary cold sore virus infection usually occurs before the age of 20 years, with infants as young as 6 months becoming infected. The ways of getting infection are multiple: direct contact with the virus via saliva, from kissing or sharing personal items, or by skin-to-skin contact. It is important to know that a person with the virus can be contagious at any time, with or without having a cold sore outbreak. The herpes virus enters the body through a break in the skin around or inside the mouth. An outbreak is then experienced within just a few days. After this initial infection, the virus stays dormant inside the nerve cells of the face. In approximately one-third of people, the virus can “wake up” or reactivate becoming recurrent herpes. When reactivation occurs, the virus travels down the nerves to the skin where it causes cold sores: blisters around the lips, in the mouth or, in about 10% of cases, on the nose, chin, or cheeks.