What Is Carcinogenesis and How Does It Cause Cancer?

Carcinogenesis is the biological process by which normal cells gradually transform into cancer cells, driven by accumulating genetic and epigenetic changes that override the body’s built-in safety controls. It is not a single event but a multistep journey that can unfold over years or even decades, progressing through recognizable stages as cells acquire the ability to grow without restraint, dodge immune detection, and eventually invade distant tissues. Understanding how this process works reveals why cancer is so difficult to prevent and treat, and why researchers are increasingly focused on interrupting the process before a full-blown tumor ever forms.

The Three Classical Stages

Scientists have long described carcinogenesis as unfolding in three overlapping stages: initiation, promotion, and progression. Initiation is the opening act. A cell’s DNA sustains damage, sometimes from a single exposure to a cancer-causing agent, and the resulting mutation becomes permanent. In classic experiments on mouse skin, researchers showed that initiation could stem from a mutation in just one critical gene in only a handful of cells, and once it happened, the change was irreversible.¹ An initiated cell looks and behaves normally, though. It sits quietly, waiting for the right conditions to start misbehaving.

Promotion is what provides those conditions. Repeated exposure to substances or signals that spur cell division, even ones that are not themselves capable of damaging DNA, coaxes the initiated cell to multiply. This creates clusters of abnormal but still benign growths. The key distinction is that promotion requires ongoing stimulation; remove the promoter and the growths can stall or even shrink. Progression is the final, most dangerous phase, where some of these benign clusters pick up additional mutations, become genetically unstable, and acquire truly malignant traits: the ability to invade surrounding tissue, recruit their own blood supply, and seed new tumors elsewhere in the body.¹

This three-stage framework is a simplification, of course. In a living person, the stages blur together and overlap, influenced by everything from inherited genetic susceptibility to the bacteria living in your gut. In colorectal cancer, for instance, specific microbes have been linked to each phase: one species produces a toxin that directly damages DNA (initiation), another drives inflammation and cell proliferation (promotion), and a third helps tumors dodge the immune system and spread (progression).²

What Goes Wrong in the DNA

At its core, carcinogenesis is a story about genes losing their proper regulation. Two broad categories of genes are involved. Oncogenes are genes that, when switched on or amplified beyond normal levels, push cells to grow and divide aggressively. Tumor-suppressor genes do the opposite: they act as brakes, halting division when something is wrong or triggering the cell to self-destruct if damage is too severe. Cancer typically emerges when oncogenes are overactivated through mutations, gene duplications, or chromosomal rearrangements, while tumor-suppressor genes are simultaneously knocked out.³ Think of it as a car with both a jammed accelerator and cut brake lines.

The mutations themselves come from many sources. Carcinogens, whether chemical, physical, or biological, damage DNA in various ways: breaking one or both strands of the double helix, attaching chemical groups that distort its structure, or creating reactive oxygen molecules that chew through genetic material.⁴ Most cancer-causing chemicals work by binding directly to DNA, forming what researchers call DNA adducts, which are essentially molecular scars that corrupt the genetic code when the cell tries to copy itself.⁵

Your cells are not defenseless against this. Elaborate repair systems patrol the genome constantly, fixing thousands of lesions per cell per day. But when the genes encoding those very repair systems are themselves damaged or inherited in a faulty form, the mutation rate across the genome climbs steeply. Hereditary breast and ovarian cancers linked to BRCA1 and BRCA2 mutations are a well-known example: those genes normally help repair a particularly dangerous type of DNA break, and when they fail, mutations pile up far faster than normal.⁶ Hereditary colon cancer syndromes follow a similar pattern, with defective mismatch-repair genes allowing copying errors to accumulate unchecked.⁶

Epigenetic Silencing Without Changing the Code

Not every cancer-driving change involves a mutation in the DNA sequence itself. Cells also regulate genes through epigenetic marks, chemical tags that sit on top of the DNA and determine which genes are active and which are muted. One of the most common changes in cancer cells is the addition of methyl groups to the promoter regions of tumor-suppressor genes, effectively silencing them without altering a single genetic letter.⁷ The gene is still physically intact, but the cell can no longer read it.

This epigenetic silencing of tumor suppressors is thought to be an early, driving event in cancer development rather than a late consequence.⁸ That matters because it means a cell can start behaving in precancerous ways long before it accumulates the kind of dramatic mutations you would see in a late-stage tumor. It also opens a therapeutic window, because unlike mutations, epigenetic marks are potentially reversible. Some cancer drugs already target the enzymes that place or maintain these silencing marks.

Viruses That Hijack the Cell

Not all carcinogenesis starts with a chemical exposure or a random copying error. Certain viruses, known as oncoviruses, can push cells toward cancer through their own toolkit of molecular tricks. These viruses produce proteins that activate the host cell’s oncogenes or shut down its tumor suppressors. They can also trigger genomic instability, introducing point mutations, deletions, and DNA damage that accumulate over time. And they interfere with normal cell-life processes like programmed cell death, effectively keeping infected cells alive and dividing when they should have been eliminated.⁹ Human papillomavirus driving cervical cancer and hepatitis B and C viruses driving liver cancer are well-known examples where a viral infection sets the carcinogenic process in motion.

Chronic Inflammation as Fuel

A short burst of inflammation is the body’s healthy response to injury or infection. Chronic, smoldering inflammation is another story entirely. When inflammatory signals persist for months or years, they create an environment that nudges cells toward malignancy. Inflammatory molecules like TNF-α and IL-6 generate reactive oxygen and nitrogen species that can directly damage DNA. They also promote unchecked cell proliferation, stimulate the growth of new blood vessels to feed expanding tissue, and encourage changes that let cells break away from their neighbors and start migrating.¹⁰

This is why chronic inflammatory conditions, from long-standing ulcerative colitis to persistent hepatitis infections, are recognized risk factors for specific cancers. The inflammation itself is part of the carcinogenic process, not just a bystander.

How Tumors Dodge the Immune System

Your immune system is remarkably good at detecting and destroying abnormal cells. According to a widely accepted model, the immune system routinely eliminates malignant precursors and holds microscopic clusters of abnormal cells in check, preventing them from growing into detectable tumors. Cancer only emerges when neoplastic cells acquire changes that let them escape this surveillance.¹¹

Tumors use several strategies to pull this off. They can camouflage themselves by downregulating the surface markers that immune cells use to recognize threats. They can coerce the immune system by secreting signals that exhaust or deactivate T cells, the immune system’s primary cancer killers. And they can shield themselves from immune attack through cytoprotective mechanisms that make them resistant to being killed even when immune cells do find them.¹¹ Tumors also reshape their local environment by hoarding nutrients that immune cells need and accumulating metabolic byproducts that suppress immune function.¹²

Building a Blood Supply and Rewiring Metabolism

A cluster of abnormal cells can only grow so large before it runs into a supply problem. Without its own blood vessels, a tumor is limited to roughly the size that can be fed by diffusion alone, typically just a millimeter or two across. To grow beyond that, tumors must flip what researchers call the “angiogenic switch,” tipping the balance from signals that keep blood vessels at bay toward signals that actively recruit new vessels into the growing mass.¹³ This transition marks a critical turning point: a dormant, avascular cluster becomes a vascularized tumor capable of rapid expansion and, eventually, malignant progression.

Cancer cells also rewire their internal metabolism in distinctive ways. Normal cells rely heavily on oxygen-dependent energy production when oxygen is available. Cancer cells, even when surrounded by plenty of oxygen, preferentially gobble up glucose and ferment it into lactate, a far less efficient process in terms of raw energy yield.¹⁴ This quirk, known as the Warburg effect, seems counterintuitive until you consider that rapid glucose consumption generates the raw building materials a fast-dividing cell needs to construct new membranes, DNA, and proteins. Whether this metabolic shift is a consequence of other cancerous changes or might itself be a trigger for transformation remains an open debate among researchers.¹⁵

Becoming Immortal

Normal cells have a built-in countdown clock. Every time a cell divides, the protective caps on the ends of its chromosomes, called telomeres, get a little shorter. After a set number of divisions, the telomeres become critically short and the cell stops dividing or self-destructs. Cancer cells sidestep this limit, most commonly by reactivating an enzyme that rebuilds telomeres, allowing them to divide indefinitely and achieve what amounts to biological immortality.¹⁶ Without this step, even a cell with every other cancerous trait would eventually run out of road.

Invasion and the Spread to Distant Organs

A tumor that stays put is dangerous but treatable. The real lethality of cancer comes when cells break free and colonize distant sites, a process called metastasis. Before cells can travel, many undergo a shift in identity known as epithelial-mesenchymal transition, where they shed the sticky, organized characteristics of their tissue of origin and acquire traits more suited to movement: increased mobility, the ability to burrow through surrounding tissue, and greater resistance to death signals.¹⁷

Even so, metastasis is astonishingly inefficient. Circulating tumor cells face a gauntlet of obstacles: they must survive the turbulence of the bloodstream, infiltrate foreign tissue, evade immune defenders in a new location, find a supportive niche, and then lie dormant as seeds until conditions allow them to start growing again.¹⁸ The vast majority of cells that enter the bloodstream die before accomplishing any of this. Those that succeed often do so with help: tumor-derived extracellular vesicles, tiny membrane-bound packages released by cancer cells, can travel ahead to distant organs and remodel the local environment, essentially preparing the soil before the seeds arrive.¹⁹ Different integrin proteins on the surface of these vesicles appear to direct them to specific organs, which may help explain why certain cancers preferentially spread to particular sites like the lungs or liver.¹⁹

Tumor Evolution Inside Your Body

A growing tumor is not a uniform mass of identical cells. It is an evolving population, subject to the same Darwinian principles that shape species in the wild. Genetic variation between cancer cells provides the raw material, and selection pressures, including limited oxygen, immune attack, and eventually chemotherapy, favor the cells best equipped to survive.²⁰ This is why a tumor biopsied in one spot can look genetically different from a biopsy taken a centimeter away, and why treatments that wipe out the dominant population of cancer cells sometimes leave behind resistant subclones that drive a relapse. The heterogeneity within a single tumor is one of the biggest obstacles to lasting cures.

Why Aging Is the Strongest Risk Factor

If carcinogenesis requires the accumulation of multiple hits over time, it makes sense that cancer is overwhelmingly a disease of aging. Roughly two-thirds of cancers are diagnosed in people aged 60 and older.²¹ Aging contributes in almost every way imaginable: somatic mutations accumulate with each passing year, DNA repair machinery becomes less efficient, telomeres shorten, epigenetic patterns drift, and the immune system gradually loses its ability to detect and eliminate abnormal cells. On top of all this, aging tissues develop chronic, low-grade inflammation and remodel their extracellular environment in ways that make them more hospitable to emerging tumors.²¹

This explains the long latency period that characterizes most cancers. A carcinogenic exposure at age 25 may not produce a detectable cancer until age 60 or beyond, because the process requires decades of additional mutations, epigenetic shifts, and microenvironment changes before a fully malignant tumor emerges.

The Physical Side of the Tumor Neighborhood

Carcinogenesis is not purely a genetic and chemical affair. The physical properties of the tissue surrounding a tumor play an active role. As tumors grow, the extracellular matrix, the scaffolding that gives tissues their structure, tends to become stiffer. That increased stiffness is not just a passive consequence of the growing mass; it actively promotes cancer progression by triggering mechanical sensors on cell surfaces that switch on growth and migration programs.²² Cancer cells can also remodel the matrix around them, softening or stiffening it in ways that further support their spread.²³ This mechanical dimension of carcinogenesis has practical implications: stiffened tissue can also act as a barrier to drug delivery, physically blocking therapeutic molecules from reaching their targets within the tumor.

Intercepting Cancer Before It Forms

Because carcinogenesis unfolds over such a long timeline, there is a growing interest in intercepting the process at its earliest stages rather than waiting to treat an established cancer. One emerging strategy uses immune-modulating approaches, including cancer vaccines, to boost the immune system’s ability to recognize and destroy transforming cells while the immune microenvironment is still relatively favorable and the number of abnormal cells is small.²⁴ The logic is straightforward: it is far easier to eliminate a handful of premalignant cells than to fight an entrenched, heterogeneous tumor that has already learned to evade the immune system.

For cancers like lung cancer, researchers are working to map the molecular drivers of premalignant lesions, the earliest visible abnormalities that precede invasive cancer. The goal is to develop biomarkers that can identify people harboring these lesions, assess the risk that a given lesion will progress, and find therapeutic targets to stop it before it does.²⁵ This field is still young, but it represents a fundamental shift in thinking: from treating cancer as a disease you react to, toward treating carcinogenesis as a process you can interrupt.

Peto’s Paradox and What Evolution Can Teach Us

If cancer arises from accumulated mutations across dividing cells, you would expect very large, long-lived animals to get cancer far more often than humans. They have vastly more cells and more years for mutations to accumulate. Yet they do not. This observation, known as Peto’s paradox, suggests that natural cancer-suppression mechanisms have evolved independently many times across the animal kingdom, and that some species suppress cancer far more effectively than humans do.²⁶ Animals with a thousand times more cells than humans do not show a corresponding increase in cancer rates, implying their cells have mechanisms that are roughly a thousand times better at preventing or stopping the carcinogenic process.²⁶

Studying these species is an active area of research. Some large-bodied animals appear to have extra copies of tumor-suppressor genes. Others may have more aggressive DNA repair or more sensitive apoptotic responses that eliminate precancerous cells before they gain a foothold.²⁷ Understanding how evolution solved the cancer problem in other species could eventually inform new strategies for preventing or disrupting carcinogenesis in humans, though translating those insights into therapies remains a long way off.