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's waist, but in rare cases, it can cause a person to get cold sores.

Once a person is infected with HVS-1, a number of things can cause cold sores. A bad cold, a case of the flu, or any sort of disease that affects the immune system can lead to the appearance of cold sores. Extremely chapped or sunburned lips can increase the chance of a cold sore outbreak. Women who are pregnant or menstruating also have an increased risk of cold sores.

But cold sores are not just caused by physical factors. Emotional stress or prolonged upset is another factor that can bring about a cold sore outbreak. Another factor that has been seen to increase the risk of an outbreak is physical stress or fatigue. All of these stress factors weaken the immune system, making it more difficult for the body to fight the outbreak.

Many people experience cold sores only a few times in their life, while others have them much more regularly. There is no cure for cold sores. And with so many different causes that people have no control over, treatment and therapy by trained professionals are often the best ways to manage troublesome outbreaks.