University of Southern California - Health Sciences Jul 9 2025 The herpes simplex virus type 1 (HSV-1), which affects almost two-thirds of the world's population and is generally associated with oral herpes, may cause painful cold sores or fever blisters around the mouth.  Yet, when genetically engineered to fight cancer, the virus may also play an important role in treating advanced melanoma, skin cancer that has spread to other parts of the body, according to phase 1-2 clinical trial results