Home Remedies for Cold Sores Cold sores are defined as small lesions, which are filled with fluid and appear on various parts of the body, especially near the lips. These lesions often form clusters. And when they break, they form a crust on the left over sore. This is mainly caused due to the infection by the herpes virus. Exposure to sun, fever, hormonal changes and stress are some of the important causes of the occurrence of the cold sores.