Herpes labialis, commonly known as cold sores, is primarily caused by Herpes simplex virus type 1 (HSV-1). This virus establishes latency in the trigeminal ganglion and reactivates to cause recurrent vesicular lesions on the lips and perioral area. Unlike Varicella-zoster virus, which causes shingles, or Cytomegalovirus and Epstein-Barr virus, HSV-1 specifically targets the oral mucosa and adjacent skin. The clinical presentation of grouped vesicles on an erythematous base is characteristic of HSV-1 infection. Effective diagnosis and management focus on this viral etiology.

Reference: Clinical Microbiology Made Ridiculously Simple, Mark Gladwin, Edition 7.