Cold sores are caused by a virus, the herpes simplex virus or HSV. There are two kinds of herpes simplex virus, called type 1 and type 2. While both types can cause cold sores, type 1 is the variety usually responsible. Type 2 HSV is found in genital herpes and is much more rare than type 1 HSV.

HSV-1 is the most common cause of cold sores. Most people are exposed to HSV-1 between the ages of three to five. There are many ways that a person can become infected by HSV-1, including among them coming into close contact with a person who has a cold sore, ingesting contaminated fluids that have been sneezed or coughed into the air, or by coming into contact with objects that a person with a cold sore touched or used. Although most people are first infected with the HSV-1 virus early in life, they do not get their first cold sore until after puberty. Once someone is infected with HSV-1, the virus remains with them for the rest of their lives.

HSV-2 is much less common than HSV-1. It primarily causes genital herpes. HSV-2 usually only affect body parts that are below a person