The cancer, Merkel cell carcinoma, tends to occur most often on the sun-exposed areas of the body like the face, the head and the neck. Although it is rare, its incidence tripled from 1986 to 2001, and it now accounts for an estimated 1,200 cases in this country each year, the National Cancer Institute says.

The virus was discovered by a University of Pittsburgh team that includes Dr. Patrick S. Moore and his wife, Dr. Yuan Chang. In 1994, when they were at Columbia University, Dr. Moore and Dr. Chang discovered human herpes virus 8, which causes Kaposi's sarcoma, the most common malignancy in AIDS patients.

Until the advent of transplant surgery and AIDS, Kaposi's sarcoma and Merkel cell carcinoma typically affected people older than 65. Now those people have been joined as the most frequent Kaposi's and Merkel cell sufferers by those whose immune systems have been compromised by AIDS or organ transplant drugs.

The newly discovered virus belongs to the polyoma family, which scientists have studied for more than 50 years because other members of the family have been found to produce cancers in animals. Although polyoma viruses have been suspected of causing human cancers, conclusive proof has been lacking.

The Pittsburgh scientists call the new virus Merkel cell polyoma virus. In a report published online by the journal Science, they said that while they suspected that it caused Merkel cell skin cancer, more work was needed to prove it.

''We can say we have a culprit with the smoking gun at the scene of the crime, but that still doesn't mean he's guilty,'' Dr. Moore said in a telephone interview.