Tumors form when cells in your body divide without stopping. Normally, your cells grow, divide, and die on a predictable schedule controlled by your DNA. When the genes that manage this process get damaged or switched off, cells keep multiplying and pile up into a mass of tissue. About 90% of tumors arise from random mutations acquired over a lifetime, while only 5% to 10% trace back to genes you inherited at birth.
What damages those genes in the first place varies widely, from tobacco smoke to chronic infections to simple bad luck during routine cell division. Understanding these causes helps explain why tumors become more common with age and why some are harmless while others turn dangerous.
How Normal Cell Growth Goes Wrong
Two families of genes keep cell division in check. The first, called growth-promoting genes, act like an accelerator pedal. They tell cells when to grow and divide. The second group, tumor suppressor genes, act like brakes. They slow division down and trigger cell death when something goes wrong with the DNA.
A tumor can start when either system breaks. If a growth-promoting gene gets stuck in the “on” position through a mutation, the cell receives a constant signal to divide. This happens, for example, when a specific growth signal gene called KRAS mutates and locks into an always-active state. Alternatively, if both copies of a brake gene get knocked out, the cell loses its ability to stop dividing or self-destruct. The key insight is that losing one copy of a brake gene isn’t enough on its own. You carry two copies of every gene, and typically both must be disabled before the brakes fail completely.
This explains why most tumors take years to develop. A single mutation rarely causes a tumor by itself. Cells need to accumulate several specific mutations before growth becomes truly uncontrolled. Research suggests that benign tumors (noncancerous growths) typically result from two or three of these key mutations, while malignant tumors (cancers) require around four, plus additional mutations that allow them to spread.
Mutations You Inherit vs. Mutations You Acquire
Most people who develop a tumor weren’t born with a genetic predisposition. Roughly 90% of tumors come from “sporadic” mutations, meaning DNA errors that accumulate in individual cells throughout life. These can happen during normal cell division (your body copies about 3 billion DNA letters every time a cell splits, and mistakes slip through) or from environmental exposures.
The remaining 5% to 10% of tumors are linked to inherited gene variants. In these cases, a person is born with one already-damaged copy of a tumor suppressor gene in every cell. They still need a second hit to that gene before a tumor forms, but starting with one copy already out means it takes less bad luck to get there. This is why certain cancers cluster in families and why people with hereditary cancer syndromes often develop tumors at younger ages than average.
Environmental and Chemical Triggers
The International Agency for Research on Cancer currently classifies 135 agents as confirmed human carcinogens. These include tobacco smoke, ultraviolet radiation, alcohol, asbestos, processed meat, and certain industrial chemicals. Another 98 agents are classified as probable carcinogens and 326 as possible carcinogens.
These substances cause tumors through a common mechanism: they damage DNA. Tobacco smoke, for instance, contains dozens of chemicals that directly alter the structure of DNA in lung cells. Ultraviolet radiation from the sun causes specific types of DNA damage in skin cells. Alcohol is broken down in your body into a compound that can bind to DNA and cause errors during replication. Over years of repeated exposure, these small injuries add up, increasing the odds that the right combination of mutations will land in the right genes.
Infections That Drive Tumor Growth
About 12% of all cancers worldwide, roughly 2.3 million new cases in 2020, are caused by infections. The most significant culprits are a stomach bacterium (H. pylori, responsible for about 36% of infection-related cancers), human papillomavirus or HPV (31%), hepatitis B (16%), and hepatitis C (7%).
These pathogens promote tumors through different routes. Some viruses insert their own genetic material directly into your DNA, disrupting normal gene function. Others cause persistent inflammation that damages cells over decades. HPV, for example, produces proteins that disable two of the cell’s most important tumor suppressors, effectively removing the brakes on cell division. Hepatitis B and C viruses cause chronic liver inflammation that leads to repeated cycles of cell death and regrowth, each cycle carrying a risk of new mutations. HIV doesn’t cause tumors directly but weakens the immune system’s ability to catch and destroy abnormal cells, raising the risk of several cancer types.
Chronic Inflammation as a Catalyst
Inflammation is your body’s normal response to injury or infection, but when it persists for months or years, it creates conditions that favor tumor development. Immune cells at the site of chronic inflammation release reactive molecules (essentially biological bleach) that can damage the DNA of nearby healthy cells. Over time, this ongoing chemical assault increases the mutation rate in the tissue.
The relationship runs in both directions. Once early tumor cells appear, they can hijack inflammatory signals to fuel their own growth. Activated cancer-promoting genes increase the production of chemical messengers that recruit immune cells, and those immune cells, paradoxically, release growth factors that help the tumor expand. Loss of the p53 protein, one of the cell’s most critical tumor suppressors, amplifies inflammatory signaling even further, creating a feedback loop between inflammation and uncontrolled growth.
This is one reason conditions that cause long-term inflammation, such as inflammatory bowel disease, chronic hepatitis, and certain autoimmune disorders, are associated with higher tumor risk in the affected tissues.
How Obesity Changes the Cellular Environment
Excess body fat doesn’t just store energy. It actively reshapes the hormonal and metabolic environment in ways that promote tumor growth through several overlapping pathways.
Enlarged fat tissue produces low-grade, constant inflammation. As fat cells grow in size and number, they attract immune cells that release inflammatory molecules, mimicking the kind of chronic inflammation described above. At the same time, fat tissue produces a hormone called leptin in proportion to body size. Elevated leptin activates cell growth and survival pathways, suppresses the natural cell-death signals that would normally eliminate damaged cells, and in breast tissue specifically, enhances estrogen activity.
Obesity also commonly leads to insulin resistance, which forces the body to produce more insulin. High circulating insulin, often accompanied by elevated levels of a related growth factor called IGF-1, directly stimulates cell proliferation and blocks a cellular recycling process that normally clears out damaged components. Studies have linked this insulin-driven pathway to increased risk of liver, pancreatic, and other cancers. On top of all this, excess circulating fatty acids provide tumor cells with both fuel and raw building materials for constructing new cell membranes as they multiply.
Gene Silencing Without Mutation
Not all tumor-driving changes involve damage to the DNA sequence itself. Cells can also switch genes on or off through chemical tags that sit on top of the DNA, a system sometimes compared to sticky notes on a instruction manual. The instructions haven’t been rewritten, but certain pages are covered up or highlighted.
In tumor cells, this system frequently goes haywire. Tumor suppressor genes can be silenced when chemical tags called methyl groups accumulate on their control regions, effectively shutting them down without any mutation. Conversely, growth-promoting genes that should be quiet can be inappropriately activated when those same tags are removed. Tumor cells also show widespread loss of these tags across large stretches of DNA, which destabilizes the genome and makes further mutations more likely. Changes in how DNA is packaged around structural proteins also play a role: when packaging becomes too tight around tumor suppressor genes, those genes can’t be read, and the cell loses another layer of growth control.
These changes are particularly important because, unlike permanent DNA mutations, they’re potentially reversible, which has opened up an entire category of cancer therapies.
When the Immune System Fails to Catch It
Your immune system destroys abnormal cells every day. Most precancerous cells are identified and killed before they can form a tumor. But tumors that do develop have typically evolved ways to dodge this surveillance.
One common tactic is going invisible. Many tumor cells reduce or eliminate the surface markers that immune cells use to identify threats, making them essentially unrecognizable to patrolling T cells. Others display molecular “stand down” signals on their surface that tell approaching immune cells to back off. This is the mechanism targeted by checkpoint immunotherapy drugs.
Tumors also sabotage immune cells metabolically. They consume glucose and amino acids so aggressively that immune cells in the surrounding area are starved of fuel. They release waste products that actively suppress T cell function. Over time, the T cells that do recognize the tumor become progressively exhausted from constant, unsuccessful stimulation, losing their ability to kill and eventually becoming unable to multiply. In some brain tumors, a specific type of immune cell called tumor-associated macrophages appears to be a primary driver of this T cell exhaustion.
Why Tumor Risk Increases With Age
Age is the single strongest risk factor for most tumors. This makes sense in light of everything above: if tumors require an accumulation of multiple specific mutations, and mutations accrue with every cell division, then more years of life simply means more opportunities for the necessary combination to occur. A person’s cells may divide trillions of times over a lifetime, and each division carries a small chance of a consequential error.
Aging also brings declining DNA repair efficiency, a weakening immune surveillance system, longer cumulative exposure to environmental carcinogens, and increasing levels of chronic inflammation. These factors don’t just add up; they compound each other. A mutation that might have been repaired at age 20 is more likely to persist at age 65, in an immune environment less equipped to catch the resulting abnormal cell, in tissue that’s been exposed to decades of inflammatory damage.