Most warts are caused by the Human Papilloma Virus (HPV). This virus induces the epidermal cells into a rapid growth phase (hypertrophy). This is not, however, a cancerous process. It is usually self-limited, but can be treated by removing or killing the epidermal cells including the basal layer. Some strains of HPV, however, do cause cancer.
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Cancer (or neoplasm) of the melanocytes as a result of sun exposure. Notice the thickened epidermis with extensions into the dermis. These cells will invade deeper tissues and metastisize to all parts of the body and eventually kill the sunbather.
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