Cold sores, also known as fever blisters, are one of the most common viral skin conditions in humans, and despite their small size, they are medically significant because they are caused by a lifelong viral infection. The primary cause is the herpes simplex virus type 1 (HSV-1), although herpes simplex virus type 2 (HSV-2), which is more commonly associated with genital herpes, can also lead to cold sores, particularly through oral-genital contact. Once a person is infected, the virus does not leave the body; instead, it remains dormant in nerve cells and can reactivate later in life.

This characteristic is what makes cold sores recurring rather than a one-time illness. From a medical perspective, the infection is extremely widespread globally, with a large portion of the population carrying HSV-1 even if they have never experienced noticeable symptoms. The reason outbreaks become visible at certain times but not others is tied to the balance between viral dormancy and immune system control, which can shift depending on a range of internal and external factors.