Originally Published: 04 FEB 23 03:13 ET

Updated: 04 FEB 23 07:18 ET

By Sandee LaMotte, CNN

(CNN) -- Cancer deaths are on the decline in the United States, and the outlook for winning the war against this deadly disease is both good and bad.

In the United States, deaths from cancer have dropped 33% since 1991, with an estimated 3.8 million lives saved, mostly due to advances in early detection and treatment. Still, 10 million people worldwide lost their lives to cancer in 2020.

During the last three years, cancer has remained a leading cause of death in the world — more than Covid-19, said Dr. Arif Kamal, chief patient officer for the American Cancer Society.

Symptoms of cancer can mimic those of many other illnesses, so it can be difficult to tell them apart, experts say. Signs include unexplained weight loss or gain, swelling or lumps in the groin, neck, stomach or underarms and fever and night sweats, according to the National Cancer Institute.

Bladder, bowel, skin and neurological issues may be signs of cancer, such as changes in hearing and vision, seizures, headaches and bleeding or bruising for no reason, the institute said. But most cancers do not cause pain at first, so you can't rely on that as a sign.

"We tell patients that if they have symptoms that do not get better after a few weeks, they should visit a doctor," Kamal said. "It doesn't mean the diagnosis will be cancer, however."

Cases rise in younger people

Rather than wait for symptoms, the key to keeping cancer at bay is prevention, along with screenings to detect the disease in its early stages. That's critical, experts say, as new cases of cancer are on the rise globally.

A surprising number of new diagnoses are in people under 50, according to a 2022 review of available research by Harvard University scientists.

Cases of breast, colon, esophagus, gallbladder, kidney, liver, pancreas, prostate, stomach and thyroid cancers have been increasing in 50-, 40- and even 30-year-olds since the 1990s.

That's unusual for a disease that typically strikes people over 60, Kamal said. "Cancer is generally considered an age-related condition, because you're giving yourself enough time to have sort of a genetic whoopsie."

Older cells experience decades of wear and tear from environmental toxins and less than favorable lifestyle choices, making them prime candidates for a cancerous mutation.

"We believed it takes time for that to occur, but if someone is 35 when they develop cancer, the question is 'What could possibly have happened?'" Kamal asked.

No one knows exactly, but smoking, alcohol consumption, air pollution, obesity, a lack of physical activity and a diet with few fruits and vegetables are key risk factors for cancer, according to the World Health Organization.

Add those up, and you've got a potential culprit for the advent of early cancers, the Harvard researchers said.

"The increased consumption of highly processed or westernized foods together with changes in lifestyles, the environment ... and other factors might all have contributed to such changes in exposures," the researchers wrote in their 2022 review.

"You don't need 65 years of eating crispy, charred or processed meat as a main diet, for example," Kamal added. "What you need is about 20 years, and then you start to see stomach and colorectal cancers, even at young ages."

So how do you fight back against the big C? Start in your 20s, Kamal said.

Get a family history

Many of the most common cancers, including breast, bowel, stomach and prostate, are genetically based — meaning that if a close relative has been diagnosed, you may have inherited a predisposition to develop that cancer too.

That's why it's critical to know your family's health history. Kamal suggested that young people sit down with their grandparents and other close relatives and ask them about their illnesses — and then write it down.

"The average person doesn't actually know the level of granularity that is helpful in accessing risk," he said.

"When I talk to patients, what they'll say is, 'Oh, yeah, Grandma had cancer.' There's two questions I want to know: At what age was the cancer diagnosed, and what specific type of cancer was it? I need to know if she had cancer in her 30s or 60s, because it determines your level of risk. But they often don't know."

The same applies to the type of cancer, Kamal said.

"People often say 'Grandma had bone cancer.' Well, multiple myeloma and osteosarcoma are bone cancers, but both of them are relatively rare," he said. "So I don't think Grandma had bone cancer. I think Grandma had another cancer that went to the bone, and I need to know that."

Next, doctors need to know what happened to that relative. Was the cancer aggressive? What was the response to treatment?

"If I hear Mom or Grandma was diagnosed with breast cancer at 40 and passed away at 41, then I know that cancer is very aggressive, and that changes my sense of your risk. I may add additional tests that aren't in the guidelines for your age."

Cancer screening guidelines are based on population-level assessments, not individual risk, Kamal said. If cancer (or other conditions such as heart disease, diabetes, Alzheimer's, or even migraines) runs in the family, you become a special case and need a personalized plan.

"And I will tell you the entire scientific community is observing this younger age shift for different cancers and is asking itself: 'Should guidelines be more deliberate and intentional for younger populations to give them some of this advice?' "

Screen yourself

If your family history is clear of cancer, that lowers your risk — but doesn't remove it. You can decrease the likelihood of cancer by eating a healthy, plant-based diet, getting the recommended amount of exercise and sleep, limiting alcohol consumption and not smoking or vaping, experts say.

Protecting yourself from the sun and tanning beds is key, too, as harmful ultraviolet rays damage DNA in skin cells and are the prime risk factor for melanoma. However, skin cancer can show up even where the sun doesn't shine, Kamal said.

"There's been an increase of melanoma that's showing up in non-sun-exposed areas such as the underarm, the genital area and between the toes," he said. "So it's important to check — or have a partner or dermatologist check — your entire body once a year."

Skin check: Take off all your clothes and look carefully at all of your skin, including the palms, soles of feet, between toes and buttocks and in the genital area. Use the A, B, C, D, E method to analyze any worrisome spots and then see a specialist if you have concerns, the American Academy of Dermatology advised.

Also see a dermatologist if you have any itching, bleeding or see a mole that looks like an "ugly duckling" and stands out from the rest of the spots on your body.

Get vaccinated if you haven't: Two vaccinations protect against cervical and liver cancers, and others for cancers such as melanoma are in development.

Hepatitis B is transmitted via blood and sexual fluids and can cause liver cancer and cirrhosis, which is a scarred and damaged liver. A series of three shots, starting at birth, is part of the US recommended childhood vaccines schedule. Unvaccinated adults should check with their doctor to see if they are eligible.

The HPV vaccine protects against several strains of human papillomavirus, the most common sexually transmitted infection, according to the US Centers for Disease Control and Prevention.

Human papillomavirus can cause deadly cervical cancer as well as vaginal, anal and penile cancer. It can also cause cancer in the back of the throat, including the tongue and tonsils.

"These HPV-related head and neck cancers are more aggressive than the non-HPV-related cancers," Kamal said, "so boys as well as girls should be vaccinated."

Since the vaccine's approval in 2006 in the US for adolescents ages 11 to 13, cervical cancer rates have declined by 87%. Today, the vaccine can be given through age 45, the CDC said.

Breast self-exams: Breast cancer is the most common type of cancer diagnosed worldwide, according to the WHO, followed by lung, colorectal, prostate, skin and stomach cancers.

Both men and women can get breast cancer, so men with a family history should be aware of the symptoms as well, experts say. These include pain, redness or irritation, dimpling, thickening or swelling of any part of the breast. New lumps, either in the breast or armpit, any pulling in of the nipple and nipple discharge other than breast milk are also worrisome symptoms, the CDC said.

Women should do a self-exam once a month and see a doctor if there are any warning signs, the National Breast Cancer Association advised. Choose a time when the breasts will be less tender and lumpy, which is about seven to 10 days after the beginning of the menstrual flow.

Screenings and tests: At-home exams and vaccinations can save lives, but many cancers can only be detected through laboratory tests, scans or biopsies. The American Cancer Society has a list of recommended screening by ages.

Getting those done in a timely manner increases the chance for early detection and treatment, but it's still each person's responsibility to know their risk factors, Kamal said.

"Remember, guidelines are only for people at average risk," he said. "The only way someone can know whether the guidelines apply to them is to really understand their family history."

Correction: An earlier version of this story contained a quote by Dr. Arif Kamal misstating the rank of cancer as a cause of death in the world.