What is the main cause of lesions known as fever blisters?

Fever blisters, also commonly known as cold sores, are primarily caused by a viral infection, specifically the herpes simplex virus type 1 (HSV-1). This virus is highly contagious and can be transmitted through direct contact with an infected individual, often through kissing or sharing utensils. Once contracted, the virus remains dormant in the body and may reactivate due to various triggers such as stress, illness, or exposure to sunlight, leading to the characteristic lesions.

Viral infections, such as HSV-1, are distinct in their mode of transmission, symptoms, and treatment compared to allergic reactions, fungal infections, or irritation from environmental factors. Allergic reactions would typically cause symptoms like hives or itching, and fungal infections are associated with different types of skin conditions, usually requiring antifungal treatment. Environmental irritants may lead to localized skin reactions, but they do not cause the specific viral lesions associated with fever blisters. Thus, viral infection is the definitive cause of these lesions, making it the correct answer.