Cancers develop because of accumulated damage to the genes that control how your cells grow and divide. Every cell in your body follows a tightly regulated cycle of growth, division, and death. When the genes managing that cycle get damaged or switched off, a cell can begin multiplying without restraint, eventually forming a tumor. About 80 percent of cancers are linked to environmental and lifestyle factors rather than inherited genetics, which means most cases trace back to decades of exposure to things that quietly injure your DNA.
How Normal Cell Growth Goes Wrong
Your body relies on two categories of genes to keep cell division in check. The first category, called proto-oncogenes, tells cells when to grow. The second, tumor suppressor genes, tells cells when to stop growing or when to self-destruct if something is wrong. Cancer typically requires damage to both systems: a growth signal gets stuck in the “on” position, and the braking mechanism fails at the same time.
When a proto-oncogene picks up a mutation, it becomes permanently active and drives the cell to keep dividing even when no growth signal is present. Meanwhile, tumor suppressor genes like p53, sometimes called the “guardian of the genome,” normally detect DNA damage and trigger either repair or cell death. A mutated version of p53 stops responding to damage, allowing a defective cell to survive and multiply instead of being eliminated. In actively dividing tumor cells, the proteins that push the cell cycle forward remain switched on continuously rather than cycling on and off as they do in healthy tissue.
The Eight Capabilities a Cancer Cell Acquires
A single mutation is rarely enough to cause cancer. Cells need to acquire several new abilities before they become truly dangerous. Researchers have identified eight core capabilities that tumor cells develop over time:
- Sustained growth signaling: the cell generates its own “grow” messages.
- Evading growth suppressors: it ignores the signals that normally stop division.
- Resisting cell death: it disables the self-destruct program that would normally eliminate it.
- Unlimited replication: it bypasses the built-in limit on how many times a cell can divide.
- Building new blood vessels: it recruits its own blood supply to feed the growing tumor.
- Invasion and spread: it gains the ability to break away and colonize distant organs.
- Rewired metabolism: it shifts how it produces energy, favoring rapid fuel consumption.
- Immune evasion: it hides from or disables the immune cells that would normally destroy it.
Acquiring all of these traits takes time, which is why cancer is overwhelmingly a disease of aging.
Why Age Is the Biggest Risk Factor
Cancer incidence increases exponentially with age, and the explanation centers on mutation accumulation. Every time a cell copies its DNA to divide, there is a small chance of error. Over a lifetime, your cells are continually exposed to DNA-damaging events, from normal metabolism to environmental exposures. Most of these errors get repaired, but some slip through. By the time you reach your 60s or 70s, many cells carry multiple mutations. If those mutations happen to land in the genes controlling growth, the cell can begin its path toward cancer.
Aging also weakens your body’s defenses against rogue cells. The same DNA damage response that prevents cancer in younger people, by forcing damaged cells into a dormant state or killing them outright, gradually becomes less efficient. Rare genetic conditions illustrate this vividly. People born with defective DNA repair machinery, such as those with Werner syndrome, develop cataracts, heart disease, diabetes, and certain cancers decades earlier than expected. Their accelerated aging mirrors what happens in all of us, just on a compressed timeline.
What Damages DNA in the First Place
Roughly 80 percent of cancers are connected to environmental factors, a broad category that includes everything you eat, breathe, drink, and are exposed to over a lifetime. The International Agency for Research on Cancer currently classifies 135 agents as confirmed human carcinogens. These range from tobacco smoke and alcohol to ultraviolet radiation, certain industrial chemicals, and specific infections.
A study led by the American Cancer Society found that 40 percent of cancer cases and nearly half of cancer deaths among U.S. adults over 30 could be attributed to modifiable risk factors: cigarette smoking, excess body weight, alcohol, physical inactivity, poor diet, and infections. Smoking alone accounts for the largest single share. Diet may play a role in 35 to 40 percent of cases, though the exact contribution is harder to pin down because dietary patterns are complex and slow-acting.
These exposures cause cancer through direct DNA damage. Tobacco smoke, for instance, contains dozens of chemicals that form bonds with DNA, distorting its structure and introducing errors during replication. UV radiation from sunlight creates specific types of DNA lesions in skin cells. Alcohol is broken down into a compound that directly damages DNA in the cells lining your mouth, throat, and liver.
Viruses That Cause Cancer
Infections account for 15 to 20 percent of cancers worldwide, roughly 1.4 million new cases every year. Seven viruses are recognized as established human cancer-causing agents: HPV (cervical and throat cancers), hepatitis B and C (liver cancer), Epstein-Barr virus (certain lymphomas and nasopharyngeal cancer), the virus behind Kaposi’s sarcoma, HTLV-1 (a type of leukemia), and Merkel cell polyomavirus (a rare skin cancer).
These viruses don’t cause cancer the way a cold virus causes a sore throat. Instead, they insert their genetic material into host cells and produce proteins that interfere with the same growth-control machinery described above. Some viral proteins directly disable p53 or other tumor suppressors. Others force the infected cell into a constant state of division, increasing the chance that additional mutations will accumulate. The cancer itself may not appear until years or decades after the initial infection.
Changes Beyond the DNA Sequence
Not all cancer-driving changes involve mutations in the DNA code itself. Cells can also be pushed toward cancer through epigenetic modifications: chemical tags added on top of DNA that switch genes on or off without altering the underlying sequence. The most studied of these is methylation, a process where small chemical groups attach to specific spots on DNA and silence nearby genes.
When methylation spreads into the control regions of tumor suppressor genes, those genes get shut down just as effectively as if they had been deleted. For some tumor suppressor genes, this epigenetic silencing is actually more common than mutation. These changes are passed faithfully from a parent cell to all of its daughter cells, meaning a single silencing event can propagate through an entire lineage of cells. The process can be triggered by chronic inflammation, certain infections, or aging itself.
Inherited Risk vs. Bad Luck
Only about 5 percent of cancers result from a single inherited gene mutation passed down through a family, like the BRCA mutations linked to breast and ovarian cancer. Another 1 percent come from rare hereditary cancer syndromes. The vast majority of cancers are polygenic, meaning they involve the interplay of many genes, each contributing a small amount of risk, combined with a lifetime of environmental exposures.
This does not mean genetics are irrelevant. Your inherited DNA determines how efficiently your cells repair damage, how quickly they metabolize carcinogens, and how effectively your immune system detects abnormal cells. Two people with the same smoking history may have very different lung cancer risks because of subtle genetic differences in their DNA repair pathways. But the key takeaway is that for most people, the accumulation of damage over time matters far more than the genetic hand they were dealt at birth.
Why Bigger Animals Don’t Get More Cancer
If cancer is about cells accumulating mutations through division, you might expect whales and elephants to develop cancer far more often than mice. They have astronomically more cells and live much longer. Yet large animals do not have proportionally higher cancer rates, an observation known as Peto’s paradox.
Part of the answer lies in cell size. Larger animals don’t just have more cells; they also have bigger cells that divide more slowly. Slower division means fewer opportunities for copying errors. These larger cells also have lower metabolic rates per unit of body mass, which reduces the internal production of DNA-damaging byproducts. When researchers factor in both cell size and division rate, the predicted cancer risk for large animals drops dramatically, largely resolving the paradox. Elephants have also evolved extra copies of the p53 tumor suppressor gene, giving their cells additional layers of protection that smaller, shorter-lived species never needed.
The Global Picture
Globally, new cancer cases have more than doubled since 1990, reaching 18.5 million in 2023. Cancer deaths reached 10.4 million that same year. Breast cancer is the most commonly diagnosed cancer worldwide, while lung cancer remains the leading cause of cancer death. The majority of people affected live in low- and middle-income countries, where access to early detection and treatment is limited. By 2050, cancer deaths are projected to exceed 18 million annually, an increase of nearly 75 percent from current levels, driven largely by aging populations and the spread of lifestyle risk factors like tobacco use and processed diets into developing regions.