Merkel cell carcinoma (MCC) is a rare, aggressive skin cancer. It usually develops as a single, painless, bump on sun-exposed skin. The bump may be skin-colored or red-violet, and tends to grow rapidly over weeks to months. It may spread quickly to surrounding tissues, nearby lymph nodes, or more distant parts of the body. Factors associated with developing MCC include increasing age, fair skin, a history of extensive sun exposure, chronic immune suppression, and the Merkel cell polyomavirus. The Merkel cell polyomavirus has been detected in most people who have MCC.