A fever blister is a cold sore. These types of sores usually are found on the mouth, lips, chin or nostrils of a person. When sores occur inside the mouth it is not a fever blister or cold sore.
Before the fever blister or cold sore occurs, you may experience tingling or feel a hard sore spot on your lip under the skin where it will come out. This is called the prodrome stage.
Fever blisters or cold sores are caused by herpes simplex virus. Because this is a virus, it is considered a medical condition. This virus is in the same family of viruses as shingles and chickenpox.
Fever blisters or cold sores may not appear until 20 days after you have been exposed to the virus. Once the sore appears it usually last from seven to 10 days. The sore forms, opens up, oozes, then crust over.
Some things that trigger fever blisters or cold sores are stress, colds, the flu, fevers and sun exposure.
Fever blisters or cold sores are very contagious and can be spread by skin to skin contact. They are most contagious from the time they appear until they crust over.