Cancer develops when cells in your body accumulate enough DNA damage to start growing out of control. There is no single cause. Most cancers result from a combination of factors: your age, your habits, your environment, infections, and sometimes the genes you inherited at birth. Understanding these pathways can help you see where risk actually comes from and what, if anything, you can reduce.
How Cancer Actually Starts
Every cancer begins with damage to a cell’s DNA. Your cells divide billions of times over your lifetime, and each division is a chance for a copying error. Most errors get repaired. Some don’t. When enough mutations pile up in genes that control cell growth, a cell can begin multiplying without the normal brakes. This process is almost never instant. DNA damage builds up over years or decades before those cells gain the ability to grow unchecked. For ovarian cancer, for instance, the lag between the first cancer-driving mutation and a diagnosis is typically more than a decade.
Your immune system usually catches and kills abnormal cells before they become a problem. But cancer cells can learn to hide. Some display a surface protein that essentially tells approaching immune cells to stand down, switching off the attack. Others stop displaying the molecular “ID tags” that immune cells use to recognize threats, making themselves invisible. Still others release chemical signals that suppress immune activity in the surrounding tissue and recruit cells that actively block an immune response. When these evasion tactics succeed, a tumor can grow undetected.
Age Is the Biggest Risk Factor
Advancing age is the single most important risk factor for cancer. The reason is straightforward: the longer you live, the more time your cells have to accumulate mutations. Cancer incidence climbs from fewer than 26 cases per 100,000 people in age groups under 20 to more than 1,000 per 100,000 in people 60 and older. The median age at diagnosis across all cancers is 67. For specific types, the pattern holds: 63 for breast cancer, 66 for colorectal, 68 for prostate, and 71 for lung cancer.
This doesn’t mean young people can’t get cancer. They can and do. But age-related accumulation of DNA damage is the background engine driving most cases.
Tobacco
Tobacco use accounts for 25% of all cancer deaths globally and is the primary cause of lung cancer. Cigarette smoke contains over 7,000 chemicals, at least 250 of which are known to be toxic or cancer-causing. These chemicals don’t just affect the lungs. They enter the bloodstream and reach tissues throughout the body, which is why smoking is linked to cancers of the mouth, throat, esophagus, stomach, pancreas, kidney, bladder, and cervix, among others.
The damage is cumulative. Each cigarette delivers another dose of carcinogens that can cause mutations in the genes controlling cell growth. Quitting at any age reduces risk, though the years of prior exposure still matter.
Ultraviolet Radiation and Skin Cancer
Sunlight damages DNA directly. UVB rays, the ones responsible for sunburns, cause specific structural defects in DNA strands. These defects aren’t inherently good at causing mutations on their own, but a chemical change that happens afterward (a process where one of the DNA building blocks degrades into a different molecule) makes them highly error-prone when the cell tries to copy its DNA. This is the primary mutation signature found in skin cancers, including melanoma.
UVA rays penetrate deeper into the skin and reach cells in lower layers that UVB can’t easily access, including the pigment-producing cells where melanoma originates. UVA also damages DNA, though recent research shows its direct mutation-causing ability in human pigment cells is surprisingly limited compared to UVB. Still, UVA contributes to skin aging and other forms of cellular stress that can promote cancer over time. Both types of UV radiation are classified as confirmed human carcinogens.
Infections That Cause Cancer
Several viruses and at least one bacterium are established causes of cancer. These infections don’t cause cancer immediately. They create conditions, often through chronic inflammation or by interfering with genes that normally suppress tumor growth, that increase the odds over years.
- HPV (human papillomavirus): Subtypes 16 and 18 are responsible for the vast majority of cervical cancers, and HPV also drives cancers of the throat, anus, vulva, vagina, and penis. A vaccine is available and highly effective.
- Hepatitis B and C: Both cause chronic liver infection that can progress to cirrhosis and liver cancer. Hepatitis B has a vaccine; hepatitis C is now curable with antiviral treatment.
- Epstein-Barr virus (EBV): Best known for causing mono, this virus can establish a lifelong latent infection in immune cells and is associated with certain lymphomas.
- HHV-8: This virus causes Kaposi sarcoma, most commonly in people with weakened immune systems.
- Helicobacter pylori: This stomach bacterium is the only one with strong epidemiological evidence directly linking it to cancer. Chronic infection causes ongoing stomach inflammation that can lead to gastric cancer. It’s treatable with antibiotics.
Other bacteria have been associated with cancer in research studies but without the same strength of evidence. Fusobacterium nucleatum has been linked to colorectal cancer in multiple patient studies, and Chlamydia trachomatis has been associated with cervical cancer, though the causal relationship for these is still being clarified.
Obesity and Chronic Inflammation
Excess body fat does more than store energy. Fat tissue is metabolically active and, in people with obesity, produces chronic low-grade inflammation throughout the body. This persistent inflammation directly promotes tumor growth through several pathways.
People with obesity also tend to have elevated blood levels of insulin and a related hormone called insulin-like growth factor-1 (IGF-1). High insulin levels, a hallmark of insulin resistance, are associated with increased risks of colorectal, thyroid, breast, prostate, ovarian, and endometrial cancers. The insulin resistance itself often precedes type 2 diabetes, which is another independent cancer risk factor. In this way, obesity creates a hormonal and inflammatory environment that favors cancer development across multiple organ systems.
Inherited Genetic Mutations
Up to 10% of all cancers may be caused by inherited genetic changes. These are mutations you’re born with, passed down from a parent, that affect genes responsible for DNA repair or cell growth control. The most well-known examples are BRCA1 and BRCA2, which dramatically increase the risk of breast and ovarian cancer. Lynch syndrome, another inherited condition, raises the risk of colorectal and several other cancers.
Having an inherited mutation doesn’t guarantee you’ll develop cancer. It means you start life with one layer of protection already compromised, so fewer additional mutations are needed before a cell becomes cancerous. This is why hereditary cancers often appear at younger ages than their sporadic counterparts. If multiple close relatives on the same side of your family have had the same type of cancer, or cancers diagnosed unusually young, genetic counseling and testing can clarify whether an inherited mutation is involved.
Environmental and Chemical Exposures
The International Agency for Research on Cancer maintains a classification system for substances with evidence of causing cancer in humans. The strongest category, Group 1, includes agents with confirmed cancer-causing ability. Some of these are workplace-specific, like asbestos (lung cancer and mesothelioma) and benzene (leukemia). Others are more broadly encountered: outdoor air pollution, diesel engine exhaust, formaldehyde, and processed meat all carry Group 1 classifications.
UV radiation from welding is classified separately from general UV exposure due to its specific occupational risk profile. Uranium and other internally deposited radioactive materials are also confirmed carcinogens. For most people, the environmental exposures that matter most are the common ones: air pollution, secondhand smoke, radon gas seeping into homes, and UV radiation from sunlight or tanning beds.
How Long the Process Takes
One of the most important things to understand about cancer is the timeline. It is rarely a sudden event. For most tumor types, the journey from first meaningful DNA mutation to detectable cancer spans years to decades. Some cancers have especially long latency periods: ovarian cancer, as noted above, often involves more than a decade between the initial driver mutation and diagnosis.
Not all cancers follow this slow pattern. Some brain tumors show cancer-causing alterations that occurred very early in life, even during fetal development. But the general rule holds: cancer is the end result of a long accumulation of genetic damage, shaped by a combination of unavoidable biological processes (like aging and random copying errors) and external factors you may have some control over (like smoking, sun exposure, and body weight).