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

Cancer is not a single disease with one cause. It develops when normal cells accumulate damage to their DNA—the instructions that tell cells how to grow, divide, and die. This damage can happen over years or decades, and it usually takes multiple changes before a cell becomes cancerous. Some people inherit genetic mutations that increase their risk; others develop cancer from repeated exposure to harmful substances or from chronic inflammation in their body.

The key point is that cancer is not contagious and not something you "catch" from another person. It develops inside your own cells when something disrupts the normal process that keeps cell growth in check.

Key Takeaways

  • Cancer develops when cells accumulate DNA damage over time, usually from multiple exposures or inherited factors rather than a single cause.
  • Tobacco, alcohol, ultraviolet radiation, and certain infections are well-documented factors that increase cancer risk in many people.
  • Some people inherit genetic mutations that make cancer more likely, while others develop it without any family history.
  • Age is a significant factor—most cancers develop in people over 50 because damage accumulates over decades.
  • Having a risk factor does not mean you will develop cancer, and developing cancer does not mean you had a known risk factor.

Tobacco and smoking as a major cause

Smoking cigarettes is the single largest preventable cause of cancer. Tobacco smoke contains over 7,000 chemicals, at least 70 of which are known to cause cancer. When you inhale smoke, these chemicals enter your lungs and bloodstream, damaging the DNA in cells they contact. Lung cancer is the most common result, but smoking also increases the risk of cancers of the mouth, throat, esophagus, bladder, pancreas, and stomach.

Secondhand smoke—breathing in smoke from someone else's cigarette—also carries cancer risk, though lower than active smoking. Smokeless tobacco (chewing tobacco and snuff) causes cancer of the mouth and throat. The longer someone smokes and the more they smoke, the higher their risk, but cancer can develop even in light smokers or in people who quit years ago.

Alcohol consumption and cancer risk

Alcohol increases the risk of several cancers, including breast, liver, colon, and head and neck cancers. The risk rises with the amount of alcohol consumed regularly. Alcohol damages the cells lining your mouth, throat, and digestive tract, and it can interfere with how your body absorbs nutrients and processes other substances. When alcohol is metabolized in your liver, it produces acetaldehyde, a chemical that damages DNA.

The risk is higher when alcohol is combined with smoking, because the two substances together cause more damage than either alone. Even moderate drinking carries some increased risk for breast cancer in women, which is why health organizations provide specific guidelines about safe drinking levels.

Ultraviolet radiation and skin cancer

Exposure to ultraviolet (UV) radiation from the sun or from tanning beds damages the DNA in skin cells and is the primary cause of melanoma and other skin cancers. UV radiation comes in two types—UVA and UVB—and both can cause damage. The risk accumulates over a lifetime, so someone who spent years in the sun without protection faces higher risk than someone with limited sun exposure.

People with fair skin, light hair, and light eyes are at higher risk because they have less melanin, the pigment that provides some natural protection. However, people with darker skin can also develop skin cancer, and it is often diagnosed at a later stage because it is less expected and may be overlooked. Sunburns, especially severe ones in childhood, increase risk significantly.

Infections that increase cancer risk

Certain infections cause chronic inflammation or directly damage cells in ways that lead to cancer. Human papillomavirus (HPV) causes most cervical cancers and is also linked to cancers of the throat, anus, and penis. Hepatitis B and C viruses cause chronic liver inflammation that can lead to liver cancer. Helicobacter pylori bacteria, which infects the stomach, increases the risk of stomach cancer. Human T-cell leukemia virus (HTLV-1) causes a type of blood cancer.

These infections do not automatically cause cancer—most people infected with HPV or hepatitis B, for example, never develop cancer. But the chronic irritation and inflammation they cause creates an environment where cells are more likely to accumulate the DNA damage that leads to cancer. Vaccines now exist for HPV and hepatitis B, which prevent the infections that would otherwise increase cancer risk.

Inherited genetic mutations and family history

Some people inherit mutations in genes that normally protect against cancer. The most well-known are mutations in the BRCA1 and BRCA2 genes, which significantly increase the risk of breast and ovarian cancer. Lynch syndrome is an inherited condition that increases the risk of colon cancer and several other cancers. Familial adenomatous polyposis (FAP) causes hundreds of polyps in the colon that almost always become cancerous if not removed.

If multiple people in your family have had cancer, especially at younger ages, genetic testing may be recommended. However, most cancers are not hereditary—they occur in people with no family history. Having an inherited mutation means your risk is higher, but it does not mean cancer is certain. Many people with BRCA mutations never develop cancer, while others do.

Age, hormones, and other factors that increase risk

Age is one of the strongest risk factors for cancer. Most cancers develop in people over 50 because it takes time for cells to accumulate the DNA damage needed to become cancerous. Hormones also play a role—estrogen exposure over a lifetime increases breast cancer risk, which is why the risk is higher in women who started menstruating early, had children late or not at all, or used hormone replacement therapy.

Obesity increases the risk of several cancers, including breast, colon, endometrial, and pancreatic cancer. Excess body fat produces hormones and inflammatory substances that can promote cancer growth. A diet high in processed meat is linked to colon cancer. Lack of physical activity and low intake of fruits and vegetables also increase risk. Exposure to asbestos, radon, or certain pesticides in occupational or environmental settings can cause cancer years or decades after exposure.

Why some people develop cancer and others do not

Cancer is the result of both chance and exposure. Two people with identical risk factors may have very different outcomes—one may develop cancer and the other may not. This is partly because the random mutations that occur in cells are unpredictable, and partly because other factors (like immune system strength or other protective genes) vary between individuals.

Having a risk factor does not may provide cancer will develop. Many smokers never develop lung cancer. Many people exposed to asbestos never develop mesothelioma. Conversely, some people with no known risk factors develop cancer anyway. This is why cancer prevention focuses on reducing modifiable risk factors—the ones you can control—rather than on guaranteeing an outcome.

Frequently Asked Questions

Can stress cause cancer?

Chronic stress does not directly cause cancer, but it may indirectly increase risk by weakening immune function or leading to unhealthy coping behaviors like smoking or drinking. Stress also causes inflammation in the body, which is linked to cancer development. However, stress alone is not considered a primary cause of cancer.

Does artificial sweetener cause cancer?

Current scientific evidence does not show that artificial sweeteners approved for use in the United States cause cancer in humans at normal consumption levels. Some studies in animals have shown links, but animal studies do not always translate to human risk. Regulatory agencies continue to monitor this question.

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

No. Most cancers are not inherited. Even if a parent had cancer, your risk depends on the type of cancer, whether it was caused by an inherited mutation, and your own exposures and lifestyle. Genetic testing can clarify your actual risk if you have a family history of cancer at young ages or multiple relatives affected.

Can you get cancer from using your cell phone?

Research to date has not found a clear link between cell phone use and cancer. The radiofrequency energy from cell phones is different from ionizing radiation (like X-rays), which is known to damage DNA. Health organizations continue to study this, but current evidence does not support a causal relationship.

What is the difference between a risk factor and a cause?

A cause directly produces cancer; a risk factor increases the likelihood of cancer developing but does not may provide it. Smoking is both a cause and a risk factor for lung cancer. Age is a risk factor but not a cause—aging itself does not cause cancer, but the accumulation of damage over time does.