Herpes simplex virus (HSV) is a widespread virus that can infect the mouth, genitalia, and skin. While most HSV infections are mild or without symptoms, some may develop more severe side effects. One of the most common consequences of HSV infection is outbreaks of sores on the skin or mucous membranes. These sores, also known as cold or fever blisters, can be painful and take several days to a few weeks to heal. Outbreaks may be triggered by factors such as stress, illness, or exposure to the sun.