"Fever blisters" refers to small, painful sores that appear around the mouth or on the lips. They occur when a person is sick or stressed, causing a virus called herpes simplex to become active. Full definition
I'm valley Fever survivor / thriver have constant fever blisters on left side toe fungus and allergies. (globalhealingcenter.com)
Cold sores (which are also called fever blisters) are caused by type 1 of the herpes simplex virus. (health.com)
HSV - 1 is a virus that usually causes cold sores or fever blisters around the mouth, but it can also cause similar blisters anywhere on the body above the waist. (babycenter.com)