It started like a mosquito bite but later turned into a blister.

This description—starting as a small, itchy, raised lesion resembling a mosquito bite and progressing to a fluid-filled blister—is highly characteristic of herpes simplex virus (HSV) infection, most commonly HSV-1. The initial phase represents the prodromal or early inflammatory stage, where localized tingling, itching, or burning may precede visible changes. Within 12–48 hours, a small erythematous papule develops, often mistaken for an insect bite. This rapidly evolves into a tense, clear vesicle (not a pustule) filled with serous fluid rich in infectious viral particles. The vesicle may coalesce, ulcerate, crust over, and heal within 7–14 days without scarring in immunocompetent individuals. Other differential diagnoses include varicella-zoster virus (especially in primary infection), contact dermatitis with vesicular reaction, or dyshidrotic eczema—but the classic sequential progression from pruritic papule to discrete, clustered vesicles strongly favors recurrent or primary oral or labial herpes. Clinical evaluation, viral PCR testing of vesicular fluid, or point-of-care antigen assays can confirm the diagnosis.