How cancer starts in your body
Cancer begins when cells in your body start to grow in an uncontrolled way. Normally, your cells follow strict rules: they grow, divide to make new cells, and then die in an orderly process. Cancer happens when something damages the instructions inside a cell—the DNA—so that it breaks these rules. The damaged cell divides over and over, creating more abnormal cells that can spread to other parts of your body.
This damage doesn't happen all at once. It usually takes years or even decades of exposure to harmful things before enough damage builds up to cause cancer. Most people develop cancer later in life because they've had more time for this damage to accumulate. A single exposure to something harmful rarely causes cancer by itself.
Key Takeaways
- Cancer develops when DNA inside cells becomes damaged, causing them to grow and divide without stopping.
- Tobacco, excessive sun exposure, alcohol, and certain infections are among the most common causes of cancer.
- Some people inherit genes that make cancer more likely, but most cancers are not inherited.
- Obesity, physical inactivity, and certain hormones can increase cancer risk over time.
- Having a risk factor does not mean you will develop cancer—many people with risk factors never do.
Tobacco and smoking
Tobacco smoke is one of the leading causes of cancer worldwide. When you smoke cigarettes, cigars, or pipes, you inhale more than 7,000 chemicals, at least 70 of which are known to damage DNA and cause cancer. Smoking damages cells in your lungs, mouth, throat, bladder, and pancreas.
Secondhand smoke—smoke you breathe in from someone else's cigarette—also contains these harmful chemicals and can cause lung cancer even if you don't smoke yourself. Chewing tobacco and snuff (moist tobacco you place in your mouth) cause cancer of the mouth, gums, and lip. The longer someone uses tobacco and the more they use, the higher their cancer risk becomes.
Sun exposure and ultraviolet radiation
Ultraviolet (UV) radiation from the sun damages the DNA in skin cells. This is the primary cause of melanoma, the most serious type of skin cancer, and also causes basal cell and squamous cell skin cancers. The damage builds up over time, so your risk increases with total lifetime sun exposure.
People who get severe sunburns, especially as children, have a higher risk of skin cancer later. Tanning beds and sunlamps also emit UV radiation and increase skin cancer risk. Fair-skinned people are at higher risk because they have less melanin, a pigment that provides some natural protection against UV damage. However, anyone can develop skin cancer, regardless of skin color.
Alcohol consumption
Drinking alcohol increases the risk of several cancers, including cancers of the mouth, throat, esophagus, liver, colon, and breast. Alcohol damages cells and makes it harder for your body to repair DNA. It also increases levels of certain hormones that can fuel cancer growth.
The more alcohol someone drinks and the longer they drink, the higher their risk. Even moderate drinking raises cancer risk slightly. This doesn't mean one drink causes cancer, but regular consumption over years increases the chances that DNA damage will eventually lead to cancer.
Infections that cause cancer
Certain viruses and bacteria can damage cells in ways that lead to cancer. 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 and can cause liver cancer. Human immunodeficiency virus (HIV) weakens your immune system, making it harder to fight off other infections that cause cancer.
Helicobacter pylori is a bacterium that infects the stomach and increases the risk of stomach cancer. Epstein-Barr virus is linked to certain lymphomas. These infections don't automatically cause cancer—many people carry these viruses without ever developing cancer—but they do increase risk over time, especially if the infection persists for years.
Inherited genes and family history
Some people inherit genes that make cancer more likely. The most well-known are BRCA1 and BRCA2 genes, which greatly increase the risk of breast and ovarian cancer. Lynch syndrome is an inherited condition that increases colon cancer risk. Familial adenomatous polyposis (FAP) causes many polyps in the colon that almost always become cancerous if not removed.
However, inheriting a gene that increases cancer risk does not mean you will definitely get cancer. It means your risk is higher than average. Many people with these genes never develop cancer. Additionally, most cancers are not inherited—they develop because of damage that happens during a person's lifetime, not because of genes passed down from parents.
Hormones and reproductive factors
Estrogen and other hormones can increase cancer risk when levels stay high over many years. Women who take hormone replacement therapy (HRT) during and after menopause have a slightly higher risk of breast cancer. The longer someone takes HRT, the higher the risk. Pregnancy, breastfeeding, and starting menstruation later in life all lower breast cancer risk because they reduce lifetime estrogen exposure.
Obesity increases cancer risk partly because fat tissue produces estrogen. Being overweight or obese raises the risk of breast cancer (especially after menopause), colon cancer, endometrial cancer, and several others. Physical inactivity also increases cancer risk, both directly and by contributing to weight gain.
Occupational and environmental exposures
Certain chemicals and materials used in workplaces can cause cancer with long-term exposure. Asbestos causes mesothelioma and lung cancer. Benzene increases leukemia risk. Radon, a radioactive gas that seeps into some homes from the ground, is the second leading cause of lung cancer after smoking. Secondhand smoke at work or home is also an environmental exposure that increases cancer risk.
Pesticides, certain dyes, and other industrial chemicals have been linked to cancer in people who work with them regularly. The risk usually depends on how much exposure someone has and for how long. People in certain jobs—construction workers, farmers, factory workers—may have higher occupational exposures than the general population.
Frequently Asked Questions
Can you get cancer from stress?
Stress itself does not directly cause cancer, but chronic stress may increase cancer risk indirectly. Long-term stress can weaken your immune system and lead to unhealthy behaviors like smoking or drinking more. Managing stress through exercise, sleep, and social connection may help reduce overall cancer risk.
Does having a family member with cancer mean I will get it too?
Not necessarily. Most cancers are not inherited. Even if a close relative had cancer, you may never develop it. Your risk is higher than someone with no family history, but many people with a family history of cancer live their whole lives without developing it. Genetic testing can help determine if you carry inherited genes that increase risk.
Can diet alone cause or prevent cancer?
Diet is one factor among many. Eating a lot of processed meat, red meat, or foods high in sugar may increase risk slightly over time. Eating more vegetables, fruits, and whole grains may lower risk. However, diet alone does not cause or prevent cancer—it works alongside other factors like genetics, smoking, and sun exposure.
Is cancer always caused by something I did?
No. Many cancers develop without any clear cause or from factors outside your control, like inherited genes or random DNA errors that happen as cells divide. Even people who do everything right—don't smoke, limit sun exposure, exercise, eat well—can still develop cancer. Cancer is not a punishment or a sign of personal failure.
How long does it take for cancer to develop?
Cancer usually takes many years to develop. Most people who smoke don't get lung cancer until they've smoked for 20 or 30 years. Skin cancer from sun exposure often takes decades to appear. This is why cancer is more common as people age—they've had more time for DNA damage to build up.