Cold sores or blisters on the lip and mouth are caused by the herpes simplex virus, according to WebMD. The virus enters the body through the mouth or a break in the skin via infected fluid.
The herpes simplex virus that causes blisters or cold sores on the mouth or lip is typically contracted when a person comes in contact with an infected individual's saliva or touches an existing cold sore or blister, explains WebMD. For example, people who share eating utensils or razors can spread the virus. In addition, kissing a person with cold sores or blisters on the lip spreads the virus. People who touch cold sores or blisters can transfer the virus to other parts of the body as well.
Common symptoms of cold sores and blisters include pain on the lips or mouth, swollen glands on the neck, a fever and a sore throat, according to WebMD. When blisters break, some patients experience pain when clear fluid drains and ultimately crusts over. Blisters typically take up to two weeks to heal once they break open. Although cold sores typically heal on their own, some physicians recommend topical ointments or skin creams to dry out the blisters.