Cancer is caused by changes to DNA that make cells grow and divide without the normal controls. These changes can come from inherited genes, environmental exposures, infections, or simple copying errors that happen every time a cell divides. A landmark study from Johns Hopkins found that about two-thirds of cancer-driving mutations in 32 cancer types were attributable to random DNA replication errors, while the World Health Organization estimates that 37% of all new cancer cases globally are linked to preventable causes. In other words, cancer rarely has a single cause. It’s usually the result of multiple factors converging over time.
How Normal Cells Become Cancerous
Your body contains trillions of cells, and each one follows a tightly regulated cycle of growth, division, and death. Cancer starts when mutations knock out the genes that keep this cycle in check. Two categories of genes matter most here: those that tell cells to grow and those that tell cells to stop.
Growth-promoting genes, called proto-oncogenes, normally help cells divide when the body needs new tissue. When one of these genes mutates, it can become permanently switched on, like a gas pedal stuck to the floor. The cell receives a constant signal to keep dividing whether the body needs new cells or not. One well-studied example is a gene called Myc, which, when overactive, reprograms a cell’s metabolism to rapidly consume fuel and expand in size.
On the other side are tumor suppressor genes, which act as brakes. A protein called p53, for instance, normally pauses cell division when it detects DNA damage, giving the cell time to repair itself or self-destruct if the damage is too severe. When p53 is lost, damaged cells keep dividing and accumulating even more mutations. Another tumor suppressor, the retinoblastoma protein, controls a whole network of genes involved in DNA copying. Losing it removes a critical checkpoint.
A single mutation is rarely enough. Cancer typically requires several hits across different genes before a cell fully escapes the body’s controls. Researchers have identified at least ten biological capabilities a tumor needs to thrive: sustained growth signaling, evasion of growth suppressors, resistance to cell death, unlimited replication, the ability to build its own blood supply, invasion into other tissues, altered energy metabolism, evasion of the immune system, genomic instability that accelerates further mutations, and inflammation that supports tumor development. No single mutation delivers all of these. That’s why cancer usually develops over years or decades.
Random Copying Errors and “Bad Luck”
Every time a normal cell divides and copies its roughly 3 billion letters of DNA, it makes mistakes. Most are caught and repaired, but some slip through. These random replication errors are a potent and historically undervalued source of cancer-driving mutations.
A study by Cristian Tomasetti and Bert Vogelstein at Johns Hopkins found that random copying errors explained the dramatic variation in cancer rates among different tissues better than hereditary or environmental factors. Tissues where stem cells divide frequently, like the colon and lung lining, develop cancer far more often than tissues with slow-dividing cells, like bone or brain. About two-thirds of cancer mutations across 32 cancer types traced back to these unavoidable replication mistakes rather than to lifestyle or inherited genes.
This finding helps explain why cancer sometimes strikes people who do everything right: they don’t smoke, they eat well, they exercise, and they have no family history. For cancers driven almost entirely by random errors, early detection becomes the primary tool for reducing deaths, since there’s no external exposure to eliminate. For cancers where environmental factors play a larger role, prevention through behavior change remains the most effective strategy.
Preventable and Environmental Causes
Despite the role of random mutations, a substantial share of cancer is preventable. A WHO analysis of 2022 global data estimated that 7.1 million new cancer cases, roughly 37% of the total, were linked to modifiable risk factors.
Tobacco is the single largest preventable cause, responsible for an estimated 15% of all new cancer cases worldwide. In men, smoking alone accounted for 23% of new cases. Tobacco smoke contains dozens of chemicals classified as carcinogenic to humans, and they damage DNA in the lungs, throat, bladder, and other organs directly exposed to these compounds or their metabolic byproducts.
Alcohol consumption accounts for about 3% of new cancer cases globally (4% in men). Even moderate drinking raises the risk for cancers of the mouth, throat, esophagus, liver, and breast. High body mass index is another significant contributor, particularly for women, where it accounts for roughly 3% of new cases. Excess body fat drives chronic inflammation, elevates hormone levels, and creates conditions that favor tumor growth.
Physical carcinogens matter too. Ultraviolet radiation from the sun and tanning beds damages skin cell DNA directly, and the International Agency for Research on Cancer classifies UV radiation as a definite human carcinogen. Ionizing radiation from sources like radon gas in homes or medical imaging at high doses also increases risk by causing breaks in DNA strands.
Infections That Drive Cancer
Roughly 10% of all new cancer cases worldwide are caused by infections, making this the second largest preventable cause after tobacco. Several viruses are well-established cancer drivers.
Human papillomavirus (HPV) is the main cause of cervical cancer and plays a role in cancers of the anus, penis, vagina, vulva, and throat. HPV vaccines now prevent infection with the highest-risk strains, making these cancers increasingly preventable.
Hepatitis B and hepatitis C viruses both increase the risk of liver cancer. Hepatitis B is more likely to cause noticeable symptoms, while hepatitis C often progresses silently to chronic infection, liver damage, and eventually cancer. Vaccines exist for hepatitis B, and effective antiviral treatments can now cure most hepatitis C infections.
Epstein-Barr virus, which causes mononucleosis, increases the risk of nasopharyngeal cancer and certain lymphomas. HIV doesn’t directly cause cancer but weakens the immune system enough that other cancer-causing infections gain a foothold, raising the risk of Kaposi sarcoma, cervical cancer, several lymphomas, and cancers of the lung, liver, anus, and throat. A rarer virus called HHV-8 is found in nearly all Kaposi sarcoma tumors. And Merkel cell polyomavirus, discovered only in 2008, is now thought to be linked to nearly all cases of Merkel cell carcinoma, an aggressive skin cancer.
Inherited Genetic Mutations
About 5% to 10% of cancers develop from genetic mutations that are inherited from a parent and present in every cell of the body from birth. These are different from the mutations that accumulate randomly or through environmental exposure over a lifetime.
The most widely known examples are mutations in the BRCA1 and BRCA2 genes. These genes normally help repair damaged DNA. When they’re not working, mistakes accumulate inside cells and the risk of breast, ovarian, prostate, and pancreatic cancer rises significantly. People who carry these mutations don’t inevitably develop cancer, but their lifetime risk is substantially higher than average.
Inheriting a cancer-related mutation is like starting partway down the path toward cancer. You still need additional mutations to occur, but you need fewer of them because one critical safeguard is already missing. That’s why hereditary cancers tend to appear at younger ages than cancers driven purely by accumulated damage over time. Genetic testing can identify many of these inherited mutations, allowing for earlier and more frequent screening.
Why Cancer Risk Rises With Age
The majority of cancers are diagnosed in people over 60, and age is the single strongest risk factor. This isn’t simply because older people have had more time to accumulate mutations, though that’s part of it. Research shows that about half of all somatic mutations actually accumulate before a person reaches maturity, because stem cells divide rapidly during growth and development. After that, stem cell division slows considerably, and mutation accumulation decelerates with it.
What changes dramatically with age is the tissue environment. Young, healthy tissues actively suppress the expansion of mutant cell populations. Oncogenic mutations that would be harmful in youth are effectively kept in check by the surrounding tissue architecture and a process similar to natural selection at the cellular level. As tissues age, this microenvironment deteriorates. Mutations that were neutral or even disadvantageous in younger tissue can suddenly become advantageous in the altered landscape of aging tissue, allowing mutant clones to expand for the first time.
This model explains a pattern that puzzled researchers for decades: many people carry cancer-driving mutations in seemingly healthy tissue, yet cancer doesn’t develop until much later in life, if at all. The mutations alone aren’t sufficient. The aging of the tissue ecosystem around those mutations is what tips the balance. It also explains why cancer incidence doesn’t simply rise in a straight line with age but accelerates sharply after midlife, when tissue maintenance systems begin to break down more noticeably.
How These Causes Overlap
In most cancers, no single cause acts alone. A person might inherit one faulty DNA repair gene, accumulate random copying errors over decades, and then have tobacco smoke deliver the final mutations that push a cell past the tipping point. The immune system, which normally detects and destroys abnormal cells, may weaken with age or illness, allowing a precancerous cell to survive when it otherwise wouldn’t have.
This layered nature of cancer explains why risk reduction is so effective even though it can’t eliminate risk entirely. Avoiding tobacco removes the single largest external source of DNA damage. Vaccination against HPV and hepatitis B eliminates two major infectious drivers. Maintaining a healthy weight reduces the chronic inflammation and hormonal shifts that promote tumor growth. Each of these steps removes one potential layer from the process, making it less likely that enough mutations will accumulate in the right combination to trigger cancer. None of them can prevent the random copying errors that are simply part of being a biological organism with dividing cells.