Cold sores are small blisters that develop on the lips or around the mouth. They 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. The strain of HSV usually responsible for cold sores is known as HSV-1, and in rare cases, cold sores can also be caused by the herpes simplex virus type 2 (HSV-2) . Cold sores can be spread by oral sex, even if the person doesnt have visible sores. The virus can spread even if you dont see the sores, and many people who are infected with the virus that causes cold sores never develop symptoms. Once you have the virus, it stays in your body and can cause outbreaks of cold sores. 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. Some people don’t have any symptoms with the first attack, while others have flu-like symptoms and ulcers in and around the mouth.