(NewsNation) — “Margaritaville” singer Jimmy Buffett died after fighting Merkel cell skin cancer, according to his website. Buffett, 76, had been battling the disease for four years before passing away at his home in Sag Harbor, Long Island on Friday.

What is Merkel cell carcinoma? The Mayo Clinic defines it as a “rare type of skin cancer that usually appears as a flesh-colored or bluish-red nodule, often on the face, head or neck.”

The nonprofit says Merkel cell cancer most often develops in older people, and long-term sun exposure or a weak immune system may increase the risk of it.

Researchers say the rare, aggressive skin cancer tends to spread rapidly to other parts of the body. It starts in the Merkel cells, which are reportedly found at the base of the outermost layer of skin and are connected to nerve endings.

Mayo Clinic states that a common virus, Merkel cell polyomavirus, may play a role in causing cases of Merkel cell cancer. Exactly how the virus contributes to the cause is still unknown.

Buffett continued to perform during treatment for the skin cancer. The “It’s Five O’Clock Somewhere” star played his last show in early July, making a surprise appearance in Rhode Island.

Following his death, Buffett’s loved ones have requested that in lieu of flowers, donations be made to Jimmy Buffett’s Foundation Singing for Change, Brigham and Women’s Hospital, Dana Farber Cancer Institute or MD Anderson Cancer Center.