Also known as fever blisters or cold sores, fever sores are commonly an indication of chronic herpes simplex infection. An active herpes simplex infection lasts approximately one week and is highly contagious, according to Healthline.
Mouth sores are also caused by oral trauma or irritation, such as tooth brushing or biting, or by stress. Stress-related sores are often called canker sores. Rarely, oral sores may be caused by cancer, bacterial infections, AIDS, autoimmune or bleeding disorders, chemotherapy, or bacterial infection, states Healthline. People with fever sores or other oral sores should see a doctor if they experience high fevers, spreading sores or severe pain, according to WebMD.