When spring turns to summer and warm weather lures more people outside, skin cancer may be at most a distant concern. But experts said it’s important to take the risk seriously.

The ultraviolet rays in sunlight are a leading risk factor for skin cancer, which will affect one in five Americans over their lifetime. That includes melanoma, among the deadliest types. About 100,000 people are diagnosed with melanoma each year in the United States, and about 8,000 die from it annually, according to the American Cancer Society.

Fortunately, there are simple ways to reduce your risk, and to detect possible cases early while they are most curable. Therapies approved over the past 15 years have also transformed the treatment of melanoma, extending and improving the lives of patients even with late-stage cases.

Here’s what to know about melanoma, its treatments and how to protect yourself.

What is melanoma?

Melanoma is a cancer that typically starts in skin cells known as melanocytes that make the skin’s pigment. Compared with more common skin cancers that begin in squamous or basal cells, melanoma is more likely to spread to other parts of the body.

“It truly has a very aggressive behavior and biology behind it,” said Dr. Michael Davies, chairman of the melanoma medical oncology department at the University of Texas MD Anderson Cancer Center.

Most melanomas appear as flat or slightly elevated blotches of dark color on skin that has been frequently exposed to ultraviolet light, such as the scalp and face, arms, back and legs (though they can occur on areas that have never been exposed to the sun, too). In a smaller share of cases, the growth may appear as a dark- or red-colored bump and grow down into the skin, which can make it more difficult to detect.

A less common form, lentigo maligna melanoma, mostly afflicts older adults who have had significant sun exposure, and often appears as abnormally-shaped tan or brown spots on their heads or necks. An even rarer type, called acral lentiginous melanoma, occurs on the hands and feet (specifically the soles, palms, fingers, toes or nail beds) and accounts for more than half of melanoma cases in those who are not white. (The musician Bob Marley died from this type of melanoma, which was at first misidentified as a bruise.)

Melanoma can also occur in the eyes or mucosal membranes such as inside the nose or throat, but these cases are rare.

Who’s at risk? And can melanoma be prevented?

Melanoma is thought to be caused by a mix of genetic and environmental factors. One of the leading hazards is exposure to ultraviolet light, including from the sun and indoor tanning. A history of severe, blistering sunburns can raise your risk; as can living close to the Equator or at a high elevation, where the sun’s rays are more intense.

The best way to reduce your risk is to avoid unnecessary exposure to UV light. The sun’s rays are strongest between 10 a.m. and 4 p.m., so limit your time outside during those hours. Wear protective clothing and eyewear and regularly apply sunscreen of at least SPF 30.

Dr. Shanthi Sivendran, a medical oncologist and senior vice president at the American Cancer Society, also warned against using tanning lamps and beds, which significantly increase melanoma risk. Twenty states and the District of Columbia have barred minors from using tanning beds, in part because of this concern, according to the Skin Cancer Foundation. But six states (Alaska, Colorado, Iowa, Montana, New Mexico and South Dakota) do not restrict them from doing so.

People with lighter skin are more vulnerable to damage from UV light. But Dr. Sivendran said that doesn’t mean those with darker skin shouldn’t also remain vigilant. “You can get melanoma regardless of what your skin color is,” she said.

It’s also important to know if melanoma runs in your family, which can heighten your risk. And people with compromised immune systems are also more likely to develop melanoma. While about half of cases occur in people aged 66 and over, younger people can also develop melanoma.

How can I detect melanoma?

Spotting melanoma early is vital, because nearly all cases that have not spread to other parts of the body are curable. However once the disease reaches the lymph nodes or more distant organs, five-year survival rates decrease significantly.

There are no standard guidelines for skin cancer screening, but clinicians can examine your skin for abnormalities during annual checkups. Dr. Kelly Nelson, a dermatologist at the MD Anderson Cancer Center, also recommended that patients conduct their own regular head-to-toe self exams.

To recognize changes in your skin, it’s helpful to be familiar with it, Dr. Nelson said. “People who are more aware of what the skin on their backs looks like are less likely to die of melanoma than people who have no clue at all.”

“It’s this fine line of balancing between having some degree of skin awareness, but also not being worried that every mole on your body is a ticking time bomb,” she added.

To distinguish melanomas from ordinary moles or irritations, dermatologists suggest checking for the “ABCDEs”: spots that have an asymmetrical shape, a notched or scalloped border, an unusual color pattern, a diameter larger than six millimeters, or a spot that has evolved over time.

In practice, however, Dr. Nelson said that patients often have trouble making these distinctions. She recommended looking out for “ugly ducklings,” meaning abnormalities that stand out for any reason.

How is melanoma treated?

For cancers that haven’t spread, a doctor will likely cut out the growth along with a margin of surrounding skin. They also may conduct a biopsy of nearby lymph nodes to assess the risk of the cancer spreading and help the doctor decide if further treatment is needed.

While melanoma is more likely to be deadly if it reaches distant parts of the body, major advances in treatment have improved the outlook, even for those who didn’t catch their cancer early.

These include treatments that harness the immune system to fight tumors, and targeted therapies that directly attack cancer cells.

There are more innovations on the way. In February, the Food and Drug Administration approved the first cancer treatment using tumor-infiltrating lymphocytes, known as TIL therapy, for use against melanomas that have not responded to other treatments. And scientists are also testing a vaccine tailored to the specific genetic makeup of a patient’s cancer in a late-stage clinical trial.