Cold sores are mainly caused by the herpes simplex virus, or HSV, particularly the HSV-1 strain, states Mayo Clinic. Although rarely, another type of this virus known as HSV-2 also causes cold sores.
Cold sores are small, fluid-filled pustules found on the lips or the area surrounding the mouth. Individuals who develop this contagious disease, medically referred to as herpes labialis, acquire the HSV through a lesion on or around their lips.
Although HSV-1 is typically associated with cold sores and HSV-2 is usually linked to genital herpes, both pathogenic strains can infect the skin at any location and cause fever blisters. The virus is normally transmitted to other people through direct contact with a cold sore or infected fluid, notes WebMD.