Cold sores are caused by the herpes simplex virus (HSV), which is highly contagious and can be easily passed from person to person by close direct contact, such as kissing or sharing utensils, towels, or razors. The most common strain of the virus causing cold sores is herpes simplex virus 1 (HSV-1), but in rare cases, cold sores can also be caused by herpes simplex virus type 2 (HSV-2) . Both types can also cause sores on your genitals and can be spread by oral sex. Once the herpes simplex virus is in your body, it can cause outbreaks of cold sores, which are small blisters around the mouth that can be painful and last up to two weeks. Cold sore outbreaks are often triggered by exposure to hot sun, cold wind, a cold or other illness, a weak immune system, or even stress.