Jimmy Buffett died Friday following a four-year battle with Merkel cell skin cancer.

The musician kept performing while undergoing treatment for skin cancer, and Buffett's last show was a surprise 45-minute appearance at a July 2 Mac McAnally show in Portsmouth, Rhode Island, where he brought the crowd to its feet screaming when he walked out. In 2022, his struggles with a then-undisclosed health issue became visible when he was hospitalized and forced to cancel several shows.

According to a statement announcing his death, the 76-year-old "passed away peacefully" at home in Sag Harbor on Long Island, New York. He was "surrounded by his family, friends, music and dogs."

So, what exactly is Merkel cell skin cancer? What causes it and who is at most risk? Here's what to know about the rare skin condition.

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What is Merkel cell skin cancer?

According to The National Cancer Institute, Merkel cell skin cancer is a rare carcinoma in which malignant − meaning, cancerous − cells form in the skin. "Merkel cells are found in the top layer of the skin," the institute says. "These cells are very close to the nerve ending that receive the sensation of touch."

Merkel cell carcinoma, also called neuroendocrine carcinoma of the skin or trabecular cancer, forms when those Merkel cells grow out of control. It starts most often in areas where skin is exposed, especially the head and neck, as well as the arms, legs and trunk.

Merkel cell carcinoma tends to metastasize at an early stage, spreading "first to nearby lymph nodes and then may spread to lymph nodes or skin in distant parts of the body, lungs, brain, bones or other organs," according to The National Cancer Institute.

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Who is at risk of Merkel cell skin cancer?

The National Cancer Institute says risk factors for Merkel cell carcinoma include those who are older than 50 years old, male or White. According to Mayo Clinic, the risk of Merkel cell carcinoma increases as people age and while it is most common in those older than age 50, it can occur at any age.

Other risk factors, the National Cancer Institute says, include being exposed to a lot of natural sunlight, exposure to artificial sunlight from tanning beds or psoralen and ultraviolet A (PUVA) therapy for psoriasis, having a weakened immune system by disease or having a history of other types of cancer.

Merkel cell skin cancer is second to melanoma as the most common cause of skin cancer death, affecting approximately 3,000 Americans every year, per Cleveland Clinic.

What are the symptoms of Merkel cell skin cancer?

Merkel cell carcinoma usually appears on sun-exposed skin as a single lump that is fast-growing, painless, firm and dome-shaped or raised and red or violet in color, according to The National Cancer Institute. That may be the first sign of a possible Merkel cell skin cancer diagnosis.

"Most Merkel cell carcinomas appear on the face, head or neck, but they can develop anywhere on your body, even on areas not exposed to sunlight," according to the Mayo Clinic.

The Mayo Clinic suggests setting an appointment with a doctor if you notice a mole, freckle or bump that is changing in size, shape or color, growing rapidly or bleeding easily after washing your skin or shaving.

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What causes Merkel cell skin cancer?

The Mayo Clinic states that while exposure to isn't the proven cause of Merkel cell carcinoma, it is considered a risk factor.

"Researchers recently discovered that a common virus plays a role in causing most causes of Merkel cell carcinoma," Mayo Clinic states. "The virus (Merkel cell polyomavirus) lives on the skin and doesn't cause any signs or symptoms. Just how this virus causes Merkel cell carcinoma has yet to be determined. Given that the virus is very common and Merkel cell carcinoma is very rare, it's likely that other risk factors play a role in the development of this cancer."

How can I prevent skin cancer?

Tips via Mayo Clinic to prevent skin cancer:

• Avoid the sun during peak hours typically from 10 a.m. to 4 p.m.
• Protect your skin and eyes by wearing hats, tightly woven clothing and sunglasses with ultraviolet (UV) light protection.
• Apply sunscreen liberally and often, using a broad-spectrum sunscreen with an SPF of at least 30 even when it's cloudy outside. Reapply every two hours and more often if you're swimming or perspiring.
• Stay on top of changes in the skin.

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