Cold sores -- those painful, fluid-filled, blister-like lesions that appear on or surrounding the lips -- are caused by the contagious herpes simplex virus, or HSV. Two types of this virus, type 1 and type 2, can both cause cold sore-like lesions, but most cold sores are associated with type 1 infections. Cold sores often tend to recur during times of physical or emotional stress, although the exact triggers can vary among individuals.
Stress -- whether it's emotional stress, or physical stress, such as the stress caused by injury, surgery, or dental procedures -- can lead to depressed immune system function. The diminished immune response can trigger viral reactivation and a new outbreak. An article published in 2001 in the journal "Medical Hypothesis" concluded that the release of various chemicals during periods of both acute and chronic stress is at the root of immune system compromise in HSV sufferers.
Fever and Illness
During an acute illness such as the cold or flu, the body is using its full immune system armament. The defense system is fully engaged in an attempt to trap and kill the microbes causing the acute attack, and to heal the damage caused by these organisms. All the while, the latent or hidden HSV that has been kept quiet by the surveillance of an alert and fully functioning immune system has a chance to re-emerge. HSV then comes out of its sleeping phase, inducing the symptoms of itching or burning at the site where the blister-like lesion will develop. Within 24 to 48 hours a full-blown cold sore outbreak is in effect. The appearance of cold sores during the time of a fever has led to another commonly used name for cold sores: fever blisters.
Research published in 1994 in the journal "Cutis" measured HSV-1 cold sore outbreaks during the summer months. In this very large study population of 3,678 patients, roughly 10 percent were classified as having sun-induced flare-ups. However, as the heat of the summer sun increased throughout the months of June, July and August, the sunlight-induced flare-ups increased too: the percentage of patients affected by sunlight ultimately increased to 40 percent. The article speculated that solar UV exposure could induce recurrent cold sores either by lowering the immune system or by direct reactivation of the dormant virus.