‘‘Skin cancer? I didn’t think people died of that,’’ said one Seattle resident.s

They do, of course, and more of them all the time. Skin cancer is the fastest-growing form of cancer in the country - even though it’s one of the most preventable and is the target of research on many fronts, including the search for a vaccine. Scientists also just announced identification of a gene for melanoma, the most deadly type of skin cancer.

The incidence rate for melanoma - the kind that killed Maureen Reagan - nearly quadrupled in the U.S. from 1973 to 1998. The American Cancer Society estimates 51,400 Americans will be diagnosed with it this year, and 7,800 will die from it. Though the percentage of patients who die is falling somewhat with earlier detection, total deaths are rising as more people get the disease.

Skin cancer’s impact is notable also because its victims tend to be younger than those of most cancers. Average age of skin-cancer diagnosis is 49, and victims in their teens, 20s and 30s are not uncommon, says researcher Dr. John Thompson of the University of Washington and the Fred Hutchinson Cancer Research Center.

Experts aren’t certain why overall skin-cancer rates are rising, but they offer a couple of theories: People are spending more time, in skimpier clothing, under the skin-damaging sun. And the atmosphere’s thinning ozone layer allows more damaging ultraviolet rates to reach us.

Supporting the latter notion is the fact that the world’s highest rates of skin cancer occur in Australia and among Caucasians in South Africa, where the ozone layer is more depleted, says Thompson.

Whatever the reasons for skin cancer’s increase, authorities say far fewer people would die from it, or even get it, if everyone followed widely accepted prevention and early detection advice.

When detected early, melanoma is 95 percent curable; diagnosed late, it’s tough to beat, in part because this cancer can quickly spread to distant parts of the body.

Less dangerous are the non-melanoma skin cancers, basal-cell carcinoma and squamous-cell carcinoma, which almost never spread beyond the original site. However, they can invade nearby tissue and bone, and in rare instances can be fatal.

All three types originate in the skin’s top layer, the epidermis, then grow deeper if not checked by treatment - primarily, surgery; in later-stage cases, chemotherapy or radiation may follow. Melanoma begins most often in an existing mole, usually one exposed to the sun. Much more rarely, it begins in an area that’s not sun-exposed, possibly indicating that genetics plays a role.

Still, excessive sun exposure gets the blame for most skin cancer. Though scientists say a tan represents sun-damaged skin, it remains a beauty standard.

‘‘We’re up against the ‘Baywatch’ effect,’’ says Thompson. ‘‘People watch that and want to have a beautiful, tanned body.’’

Increasingly, studies indicate that too much sun in childhood and the early teens - especially repeated, severe sunburn - greatly raises the risk of getting skin cancer later. As adults, we can’t undo the sunburns we had as kids, but health experts warn parents to make sure their children avoid too much sun.