Cancer starts when cells in your body begin to grow in an uncontrolled way

Your body is made of trillions of cells. Normally, cells grow, divide, and die in an orderly process. Cancer happens when this process breaks down—cells start dividing when they shouldn't, don't die when they should, and pile up into tumors. This doesn't happen by accident. Something has to damage the instructions inside a cell (its DNA) badly enough that the cell stops following the normal rules.

That "something" is what doctors call a risk factor. A risk factor is anything that makes cancer more likely to develop. Some risk factors you inherit from your parents. Some come from your environment—the air you breathe, the sun you're exposed to, the chemicals you encounter. Some come from your own choices—smoking, drinking, diet, weight. Most cancers result from a combination of these, not from a single cause.

Key Takeaways

  • Cancer develops when damage to a cell's DNA causes it to divide uncontrollably; this damage usually builds up over years or decades, not overnight.
  • Some people inherit genes that make cancer more likely, but inheriting a gene does not mean you will definitely get cancer.
  • Common risk factors include smoking, sun exposure, alcohol, obesity, age, and certain infections like HPV and hepatitis B.
  • Most cancers are caused by a mix of factors—genetics, environment, and lifestyle choices working together—rather than one single cause.
  • Knowing your risk factors helps you understand which cancers to watch for and what changes might lower your risk.

How DNA damage leads to cancer

Every cell in your body contains DNA—the instruction manual that tells the cell how to behave. When a cell divides, it copies this manual. Usually the copy is perfect. But sometimes damage happens: a chemical burns a piece of the manual, radiation tears it, or a virus inserts bad code. If the damage hits the wrong spot—a gene that controls growth or tells damaged cells to die—the cell can start misbehaving.

One piece of damage rarely causes cancer. Your body has repair crews that fix most damage before it matters. But if damage happens again and again in the same cell, the repairs can fail. After enough hits—usually five to ten key mutations in the same cell—that cell stops listening to the body's rules. It divides when it shouldn't. It ignores the signal to die. It spreads to places it doesn't belong. That's cancer.

This is why cancer usually takes decades to develop. A 25-year-old smoker doesn't get lung cancer next year. The damage builds slowly. By age 65, after 40 years of exposure, the odds are much higher. Age itself is a risk factor because time allows damage to accumulate.

Inherited genes that raise cancer risk

Some people are born with genes that make certain cancers more likely. The most well-known are BRCA1 and BRCA2, which raise the risk of breast and ovarian cancer. Others include genes linked to colon cancer, melanoma, and other types. If your parent or sibling had cancer, especially before age 50, there's a chance you inherited a risky gene.

But inheriting a gene is not a diagnosis. A woman with a BRCA mutation has a higher lifetime risk of breast cancer—perhaps 45 to 87 percent instead of 13 percent—but most women with the mutation never get cancer. The gene loads the gun; environment and chance pull the trigger. Knowing you carry a risky gene lets you watch more closely, start screening earlier, or take preventive steps.

Genetic testing is available if your family history suggests you might carry a mutation. A genetic counselor can help you understand what the results mean and what to do next.

Environmental and lifestyle risk factors

Tobacco is the single largest preventable cause of cancer. Smoking damages lung cells directly. Chewing tobacco raises mouth cancer risk. Even secondhand smoke increases lung cancer risk over time.

Sun exposure causes most skin cancers. UV radiation damages skin cell DNA. The risk builds with total lifetime exposure, so someone who spent decades in the sun faces higher risk than someone who avoided it.

Alcohol raises the risk of cancers of the mouth, throat, esophagus, liver, and breast. The more you drink, the higher the risk. Even moderate drinking increases breast cancer risk slightly.

Obesity is linked to cancers of the breast (in postmenopausal women), colon, endometrium, pancreas, and others. Extra fat tissue produces hormones and inflammatory chemicals that can fuel cancer growth.

Infections cause some cancers. Human papillomavirus (HPV) causes most cervical cancers and some throat cancers. Hepatitis B and C viruses cause liver cancer. Helicobacter pylori bacteria raises stomach cancer risk. These infections don't always lead to cancer, but they increase the odds.

Workplace and environmental exposures matter for some people. Asbestos causes mesothelioma. Radon gas in homes raises lung cancer risk. Pesticides and other chemicals may raise risk, though the evidence is stronger for some than others.

How multiple risk factors work together

A single risk factor rarely causes cancer on its own. Instead, risk factors combine. A person who smokes and drinks has higher cancer risk than someone who does only one. Someone with a family history of breast cancer who is also obese and drinks regularly faces higher risk than someone with just the family history.

This is why two people with the same risk factors can have different outcomes. One smoker gets lung cancer at 55; another smokes for 50 years and never does. Luck and genetics both play a role. The risk factor makes cancer more likely, but doesn't may provide it will happen.

Age and cancer risk

Cancer is largely a disease of aging. Most cancers are diagnosed in people over 65. This is not because older people are more exposed to risk factors (though some are), but because cancer takes time to develop. A 30-year-old's cells have had 30 years to accumulate damage. A 70-year-old's cells have had 70 years. The longer you live, the more chances your cells have to go wrong.

This is also why screening for cancer becomes more important as you age. Catching cancer early, before symptoms appear, usually means better outcomes. Your doctor can tell you when screening makes sense for your age and risk factors.

What you can and cannot control

You cannot change your age or your genes. You cannot undo past exposures. But you can change some risk factors right now. You can quit smoking. You can limit alcohol. You can maintain a healthy weight through diet and exercise. You can protect your skin from the sun. You can get vaccinated against HPV and hepatitis B. You can have screening tests at the intervals your doctor recommends.

These changes don't may provide you won't get cancer—nothing does. But they lower your risk. And knowing your risk factors—family history, past exposures, current habits—helps you and your doctor make informed decisions about screening and prevention.

Frequently Asked Questions

Does stress cause cancer?

Stress itself has not been proven to cause cancer directly. However, chronic stress can weaken your immune system and lead to unhealthy habits like smoking or overeating, which do raise cancer risk. Managing stress is good for overall health, but it is not a cancer prevention strategy on its own.

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

No. Having a parent with cancer raises your risk, but most people with a family history of cancer never develop it. Your risk depends on which relative had cancer, at what age, what type, and whether a genetic mutation runs in your family. A doctor can help you understand your actual risk based on your specific family history.

Can diet alone prevent cancer?

Diet is one piece of cancer risk, not the whole picture. Eating more vegetables and less processed meat may lower risk slightly, but diet cannot override other major factors like smoking or heavy alcohol use. The strongest cancer prevention strategies combine diet, exercise, avoiding tobacco and excess alcohol, and appropriate screening.

Are artificial sweeteners or food additives a major cancer risk?

The evidence does not show that artificial sweeteners or most food additives cause cancer in humans at the levels people consume them. Tobacco, alcohol, obesity, and infections are far stronger risk factors. If you have concerns about specific foods, your doctor can discuss what the research actually shows.

Does living near power lines or cell towers cause cancer?

There is no strong scientific evidence that radiofrequency radiation from cell towers or extremely low-frequency radiation from power lines causes cancer. Extensive research has not found a link. If you have specific concerns about your environment, a doctor or public health official can provide information based on current evidence.