Eruption on the face caused by the herpes simplex virus (HSV type I). Herpes simplex infections are very common, and by later life almost everyone will have experienced one, usually without discomfort. One of the characteristics of the virus is that after infection it remains in the body during the patient’s lifetime, usually without causing any other manifestations of disease. However, it is also possible for the virus to produce recurrent infection, of which cold sores are an example. The venereal disease Herpes genitalis is another example of a herpes simplex infection, although it is produced by a different, HSV type II, virus. Infection with HSV type I generally occurs in the second to fourth year of life, usually without symptoms at that time. The virus then establishes itself in the nerves, from which it can later cause cold sores. Factors favourable to the development of cold sores are fever, menstruation, pneumonia and exposure to the sun. Cold sores result in characteristic small blisters where mucous membrane meets skin, particularly around the mouth and nose. The blisters are often painful and itchy, but form a scab after a few days, and the eruption disappears one to two weeks later. Treatment of cold sores is difficult. Ointments can be used to reduce the discomfort and to prevent bacterial infection in the affected area.