Herpes simplex is a viral infection caused by herpes simplex virus (HSV) that mainly affects the mouth or genital area. Oral herpes causes cold sores around the mouth or face. Genital herpes affects the genitals, buttocks or anal area. Genital herpes is a sexually transmitted disease (STD). Others get sores near the area where the virus has entered the body. They turn into blisters, become itchy and painful, and then heal. The virus can be dangerous in newborn babies or in people with weak immune systems.