How cancer starts in your body

Cancer develops when cells in your body begin to grow in an uncontrolled way. Normally, your cells follow strict rules: they grow, divide when needed, and die on schedule. Cancer happens when something damages the instructions inside a cell—the DNA—so it breaks these rules and keeps dividing instead of stopping.

That damage doesn't happen by accident. It comes from exposure to something harmful, or sometimes from a mistake your body makes on its own. Scientists have identified dozens of things that can cause this damage. Some are things you're exposed to in your environment. Others are habits or choices. A few are things you inherit from your parents, or conditions your body develops over time.

Most cancers result from a combination of factors, not just one. You might be exposed to a carcinogen—a substance that can cause cancer—but never develop the disease. Someone else might develop cancer without any obvious exposure. This is because cancer depends on both what you're exposed to and how your individual body responds.

Key Takeaways

  • Cancer starts when damage to a cell's DNA causes it to divide uncontrollably instead of following normal growth rules.
  • Common causes include tobacco smoke, excessive sun exposure, certain viruses, alcohol, obesity, and family history of cancer.
  • Most cancers result from a combination of exposures and risk factors over time, not from a single cause.
  • Some risk factors you cannot change, like age or inherited genes, while others depend on choices you make.
  • Knowing what increases your risk helps you understand your own situation and what conversations to have with your doctor.

Tobacco and smoking

Tobacco smoke is the single most preventable cause of cancer. Smoking damages the cells lining your lungs, throat, and mouth. Over time, this repeated damage can cause those cells to become cancerous. Smokers face much higher risk of lung cancer, throat cancer, and mouth cancer than people who have never smoked.

The risk doesn't stop when you quit. Former smokers still carry an elevated risk for many years, though the risk decreases over time. Secondhand smoke—breathing in smoke from someone else's cigarette—also increases cancer risk, particularly lung cancer, even though the exposure is less direct.

Smokeless tobacco, including chewing tobacco and snuff, causes cancer too. These products damage the cells in your mouth and throat where they sit. The nicotine in all tobacco products is addictive but not itself carcinogenic; the cancer risk comes from the other chemicals in tobacco.

Sun exposure and ultraviolet radiation

Ultraviolet (UV) radiation from the sun damages the DNA in skin cells. Repeated sun exposure, especially sunburns during childhood, significantly increases your risk of melanoma—the most dangerous type of skin cancer—and other skin cancers like basal cell carcinoma and squamous cell carcinoma.

The damage accumulates over your lifetime. You don't need to get a severe sunburn to increase your risk; regular unprotected sun exposure adds up. People with fair skin, red or blonde hair, and light-colored eyes are at higher risk because they have less melanin, the pigment that protects skin from UV damage.

Tanning beds emit UV radiation similar to the sun and carry the same cancer risk. Using a tanning bed before age 35 significantly increases melanoma risk. The American Academy of Dermatology recommends avoiding tanning beds entirely.

Viruses and infections

Certain viruses can cause cells to become cancerous. The human papillomavirus (HPV) causes most cervical cancers and can also cause cancers of the throat, anus, and other areas. Hepatitis B and hepatitis C viruses damage liver cells over time and can lead to liver cancer. Human immunodeficiency virus (HIV) weakens your immune system, making it harder for your body to fight off other cancer-causing viruses.

Epstein-Barr virus (EBV) is linked to certain types of lymphoma and nasopharyngeal cancer. Human T-cell leukemia virus (HTLV-1) can cause a rare type of leukemia. These viruses don't automatically cause cancer in everyone who gets infected, but they increase the risk significantly, especially if the infection persists for years.

Vaccines now exist for HPV and hepatitis B, which prevent the infections that lead to cancer. These vaccines are most effective when given before exposure to the virus, typically during childhood or early adulthood.

Alcohol and other substances

Alcohol increases cancer risk in several ways. It damages cells in your mouth, throat, and esophagus, making them more vulnerable to becoming cancerous. It also damages liver cells and can lead to liver cancer. The more you drink and the longer you drink, the higher your risk. Even moderate alcohol consumption increases risk slightly compared to not drinking at all.

Asbestos is a mineral fiber that was widely used in building materials, insulation, and other products before its dangers became clear. Inhaling asbestos fibers causes them to lodge in your lungs and can lead to mesothelioma, a cancer of the lung lining, or lung cancer itself. The risk increases with the amount and duration of exposure.

Radon is a radioactive gas that seeps into buildings from soil and rock. Long-term exposure to high radon levels increases lung cancer risk, particularly in people who also smoke. Radon testing kits can measure levels in your home.

Weight, diet, and physical activity

Obesity increases cancer risk through several mechanisms. Extra body fat produces hormones and inflammatory substances that can promote cancer growth. Obesity is linked to increased risk of breast cancer (especially after menopause), colon cancer, endometrial cancer, pancreatic cancer, and others.

Processed meat consumption—including bacon, sausage, deli meats, and hot dogs—is associated with increased colorectal cancer risk. Red meat consumption also carries some increased risk. The exact mechanisms are still being studied, but the association is consistent across many research studies.

Physical inactivity contributes to cancer risk both directly and by contributing to obesity. Regular physical activity appears to reduce cancer risk for several types of cancer. A diet high in vegetables, fruits, and whole grains and low in processed foods is associated with lower cancer risk overall.

Age and family history

Cancer risk increases with age. Most cancers develop over many years as cells accumulate damage, so older people have had more time for this damage to occur. This is why cancer is far more common in people over 65 than in younger age groups.

Some families carry inherited mutations in genes like BRCA1 and BRCA2 that significantly increase breast and ovarian cancer risk, or in mismatch repair genes that increase colorectal cancer risk. If multiple family members have had cancer, particularly at younger ages, genetic testing may help determine whether you carry a mutation that increases your risk. A genetic counselor can help you understand whether testing makes sense for your situation.

Having a family history of cancer doesn't mean you will definitely develop cancer, but it does mean your risk is higher than average. Knowing your family history helps your doctor recommend screening or preventive measures tailored to you.

Hormones and reproductive factors

Estrogen and progesterone, the hormones involved in menstruation and pregnancy, influence breast cancer risk. Women who menstruate for more years (starting periods early or going through menopause late) have higher lifetime estrogen exposure and thus higher breast cancer risk. Pregnancy, particularly at a younger age and with multiple pregnancies, actually reduces breast cancer risk.

Hormone replacement therapy (HRT), used to manage menopause symptoms, increases breast cancer risk if used for more than a few years. The increased risk decreases after stopping HRT. Oral contraceptives slightly increase breast cancer risk while being used, but the risk returns to normal after stopping.

These hormonal factors don't mean you will develop cancer, but they are part of your overall risk picture. Your doctor can discuss your individual situation and what screening or preventive steps might be appropriate.

Frequently Asked Questions

Can you get cancer from stress?

Stress itself doesn't directly cause cancer, but chronic stress may increase cancer risk indirectly. Stress can weaken your immune system, disrupt sleep, and lead to unhealthy coping behaviors like smoking or drinking. Managing stress through exercise, sleep, and social connection is beneficial for overall health.

Does living near power lines or cell towers cause cancer?

Extensive research has not found convincing evidence that radiofrequency radiation from cell towers or extremely low-frequency radiation from power lines causes cancer. The radiation from these sources is non-ionizing, meaning it doesn't have enough energy to damage DNA the way ionizing radiation (like X-rays) does.

If my parent had cancer, will I definitely get it too?

No. Having a parent with cancer increases your risk, but most people with a family history of cancer never develop the disease. Risk depends on which type of cancer, at what age your parent developed it, whether a genetic mutation runs in your family, and your own exposures and choices. Talk with your doctor about your specific situation.

Can you get cancer from artificial sweeteners or food additives?

Major health organizations including the FDA and World Health Organization have concluded that artificial sweeteners approved for use in the United States do not cause cancer at the levels people typically consume them. Some additives have been studied extensively and found safe; others are still being researched.

Does deodorant or antiperspirant cause breast cancer?

Research has not found a link between deodorant or antiperspirant use and breast cancer. This is a common concern, but studies comparing women who use these products with those who don't have not shown increased cancer risk.