Cold sores, also called fever blisters, are tiny, fluid-filled lesions that usually occur on and around your lips. These blisters are caused by the Herpes Simplex Virus (HSV). The blisters are most commonly found around the mouth area and are usually painful. They may last from 1 to 2 weeks before resolution and may slowly evolve into a crusted bump. Fever blisters spread from person to person by skin-to-skin contact. There’s no cure for HSV infection; therefore, the blisters may recur sporadically due to stress, sun exposure, or physical trauma. Antiviral treatment is quite effective in hastening the resolution of flare-ups, but for patients who are chronically plagued by herpes simplex outbreaks, maintenance antiviral therapy is recommended.