Fever blisters, also known as cold sores, are small, fluid-filled blisters that typically appear on or around the lips. They are caused by the herpes simplex virus (HSV-1) and are highly contagious. The initial infection may occur without symptoms, but subsequent outbreaks can be triggered by factors like stress, illness, or sun exposure. These blisters usually heal within 1 to 2 weeks without leaving scars. Over-the-counter treatments can help alleviate discomfort and speed up healing. It's important to avoid close contact with others during an outbreak to prevent spreading the virus, especially to those with weakened immune systems or newborns.