Cancer starts when a single cell’s DNA becomes damaged or altered in a way that lets it grow and divide without the normal controls. This doesn’t happen all at once. It typically takes multiple mutations accumulating in the same cell over years or decades before that cell becomes cancerous. Understanding how those mutations happen, and what speeds them up, is the key to understanding how you get cancer.
What Goes Wrong Inside a Cell
Your cells have two main types of genes that keep growth in check. The first type, called proto-oncogenes, act like a gas pedal. They tell cells when to grow and divide. The second type, tumor suppressor genes, act like brakes. They slow cell division down or tell damaged cells to self-destruct through a process called programmed cell death.
Cancer develops when mutations hit both systems. A proto-oncogene can mutate into what’s called an oncogene, essentially a gas pedal stuck to the floor, driving nonstop cell division. At the same time, if tumor suppressor genes mutate and stop working, the brakes fail. A cell with a stuck accelerator and no brakes will multiply out of control, and that unchecked growth is the foundation of every cancer.
One mutation alone rarely causes cancer. Most cancers require several mutations in different genes within the same cell. That’s why cancer is overwhelmingly a disease of older adults: the longer your cells have been dividing and accumulating copying errors, the greater the chance that enough mutations stack up in one unlucky cell.
Inherited vs. Acquired Mutations
Up to 10% of all cancers are caused by inherited genetic changes, according to the National Cancer Institute. These are called germline mutations because they exist in a parent’s egg or sperm cell and get passed to every cell in the child’s body. If you inherit a mutation in a tumor suppressor gene like BRCA1 or BRCA2, you’re starting life one step closer to cancer because one of your “brake” genes is already broken in every cell. It still takes additional mutations before cancer develops, but you have a significant head start in the wrong direction.
The vast majority of cancers, roughly 90% or more, come from somatic mutations. These are DNA changes that happen after conception, in ordinary cells throughout your body. They occur randomly during cell division, or they’re triggered by outside forces like tobacco smoke, radiation, or chronic inflammation. You can’t pass somatic mutations to your children because they don’t exist in your reproductive cells.
How Your DNA Gets Damaged
Every time a cell divides, it copies about 3 billion base pairs of DNA. Mistakes happen naturally during that process. Your cells have built-in repair systems that catch and fix most errors, but some slip through. Over a lifetime of trillions of cell divisions, these random copying mistakes are actually the single largest source of cancer-causing mutations.
External factors accelerate the damage. These fall into a few major categories.
Tobacco and Chemical Carcinogens
Tobacco smoke contains dozens of chemicals that directly damage DNA. These chemicals form abnormal bonds with DNA bases, distorting the structure and causing errors when the cell tries to copy its genetic code. The same principle applies to other chemical carcinogens: asbestos, certain industrial solvents, formaldehyde, and processed meat preservatives all interact with DNA in ways that increase mutation rates. The International Agency for Research on Cancer classifies these and many other substances as confirmed human carcinogens based on strong evidence linking exposure to cancer development.
Radiation
Radiation damages DNA through two distinct pathways depending on the type. Ultraviolet light from the sun is absorbed directly by DNA bases, creating abnormal bonds between neighboring bases called pyrimidine dimers. These distort the DNA strand and can introduce mutations when the cell tries to replicate. UV radiation is the primary driver of skin cancers.
Ionizing radiation, the kind from X-rays, radon gas, and nuclear material, works differently. It can knock electrons off DNA molecules directly or generate highly reactive molecules from water inside cells. These reactive molecules attack DNA from multiple angles, producing both single-strand and double-strand breaks. Double-strand breaks are especially dangerous because they’re harder for the cell to repair accurately, increasing the chance of permanent mutations.
Viruses and Infections
At least eight viruses are known to cause cancer in humans. Human papillomavirus (HPV) has at least 12 strains linked to cervical, anal, throat, penile, vaginal, and vulvar cancers. Hepatitis B and hepatitis C are leading causes of liver cancer. Epstein-Barr virus increases the risk of certain lymphomas and stomach cancer. Other cancer-causing viruses include HIV (which raises cancer risk by weakening the immune system), human herpesvirus 8 (linked to Kaposi sarcoma), human T-cell leukemia virus type 1, and Merkel cell polyomavirus.
These viruses promote cancer through different mechanisms. Some insert their genetic material directly into the host cell’s DNA, disrupting normal gene function. Others produce proteins that deactivate tumor suppressor genes or keep infected cells alive when they would normally self-destruct. HIV takes yet another route: by crippling the immune system, it removes the body’s ability to detect and destroy abnormal cells before they become cancerous.
How Chronic Inflammation Fuels Cancer
Long-term inflammation is one of the less obvious but significant drivers of cancer. When immune cells respond to ongoing irritation, infection, or tissue damage, they release reactive oxygen species, molecules that directly damage DNA and create genomic instability. In a short burst during a normal infection, this is manageable. When inflammation persists for months or years, the accumulated DNA damage becomes a serious problem.
Chronic inflammation also creates a self-reinforcing cycle. Damaged cells release signals that attract more immune cells, which release more reactive molecules, which damage more DNA. Meanwhile, inflammatory signaling molecules promote the survival and multiplication of cells that have already picked up mutations, helping them expand into larger populations of abnormal cells. Conditions that cause chronic inflammation, such as inflammatory bowel disease, chronic hepatitis, and long-term acid reflux, are all associated with elevated cancer risk in the affected tissues.
The relationship goes both directions. Once a cell acquires cancer-promoting mutations, those mutations can themselves trigger inflammatory signaling, recruiting immune cells that inadvertently create an environment that helps the tumor grow. This feedback loop between inflammation and genetic damage is one reason chronic inflammatory conditions deserve attention and management.
How Obesity Raises Cancer Risk
Excess body fat isn’t just stored energy. Fat tissue is biologically active, and it shifts the body’s hormonal environment in ways that promote cancer through several pathways.
People with obesity often have elevated levels of insulin and insulin-like growth factor-1. Both of these molecules signal cells to grow and divide, and high levels are associated with increased risks of colorectal, breast, prostate, ovarian, endometrial, and thyroid cancers. Fat tissue also produces excess estrogen, a hormone directly linked to breast, endometrial, and ovarian cancers. On top of that, fat cells release signaling molecules called adipokines. One of these, leptin, rises with increasing body fat and promotes abnormal cell growth. Another, adiponectin, helps restrain tumor growth but is less abundant in people with obesity.
Why Cancer Risk Rises With Age
Age is the single strongest risk factor for most cancers, and the reason is straightforward. Every year you’re alive, your cells accumulate more mutations from copying errors, environmental exposures, and the normal wear and tear of metabolism. By the time you reach your 60s or 70s, some cells may have collected enough mutations in the right combination of genes to begin growing uncontrollably.
Aging also weakens the body’s defenses against cancer. DNA repair mechanisms become less efficient over time. The immune system grows less capable of identifying and eliminating abnormal cells. Chronic low-grade inflammation tends to increase with age, adding another mutation-promoting force. These factors converge to explain why cancer diagnoses rise sharply after middle age, even in people with no obvious risk factors.
Putting It All Together
Cancer isn’t caused by one single thing. It’s the end result of accumulated genetic damage from a combination of sources: random copying errors during cell division, inherited mutations, environmental exposures, infections, inflammation, hormonal imbalances, and time. Most cancers require multiple hits across multiple genes before a normal cell transforms into a cancerous one. That’s why reducing the exposures you can control, such as tobacco, excess UV, alcohol, and excess body weight, meaningfully lowers your risk even though it can’t eliminate it entirely. You can’t stop your cells from making occasional copying errors, but you can avoid piling additional damage on top of those unavoidable mistakes.