Initial infection with HSV causes fever, headache, nausea, vomiting, sore throat, painful swelling and open sores on the gums, and swollen or tender glands in the neck. The first infection appears like any other viral or bacterial throat infections, and is usually not diagnosed as an HSV-1 infection. The symptoms start about 7-8 days after exposure to someone with HSV-1.
Cold sores and fever blisters appear later when the virus gets reactivated. Some common causes of reactivation of HSV virus include fever, cold, stress, dental treatment, illness, trauma to the lips, exposure to sunlight, menstruation, suppression of the immune system or allergies. Mostly the lesions of cold sores occur in the region of the lips, underneath the nose or around the chin. But in people with chronic health problems, on cancer chemotherapy, corticosteroids or immunosuppressive drugs it can cause lesions inside the mouth as well.
The initial symptoms of cold sore before the painful, fluid-filled blisters appear is a vague tingling, burning or itching, and swelling and redness in the area. The blisters appear after 24 to 48 hours. Subsequently the blisters break, and become painful sores ("cold sores"), which eventually get covered by crusts and scabs.