Cancer is a disease where cells in your body grow in an uncontrolled way

Cancer starts when cells in your body begin to divide and multiply without stopping. Normally, your cells follow a set pattern: they grow, divide to replace old or damaged cells, and then die in an orderly way. In cancer, this process breaks down. Cells keep dividing even when your body does not need new ones, and they do not die when they should. These extra cells pile up and form tumors—lumps of tissue that can damage healthy organs and tissue around them.

Not all tumors are cancer. A benign tumor grows slowly and does not spread to other parts of your body. A malignant tumor is cancer—it can invade nearby tissue and spread to distant parts of your body through the bloodstream or lymph system. This spread is called metastasis, and it is what makes cancer dangerous.

Cancer is not one disease but many. It is named for the type of cell it starts in and the organ where it begins. Lung cancer starts in lung cells, breast cancer in breast tissue, colon cancer in the colon. Each type behaves differently and responds differently to treatment.

Key Takeaways

  • Cancer occurs when cells divide uncontrollably instead of following the body's normal growth and death cycle.
  • A malignant tumor can spread to other parts of the body, while a benign tumor stays in one place and is not cancer.
  • Cancer is named for the type of cell it starts in, so there are many different kinds with different causes and treatments.
  • Most cancers develop over years as cells accumulate genetic damage from repeated exposure to risk factors.
  • Having a risk factor does not mean you will get cancer, and some people without known risk factors do develop it.

How cells become cancerous: genetic damage and mutations

Cancer develops when the DNA inside a cell gets damaged. DNA is the instruction manual that tells cells how to grow, divide, and die. When damage occurs to certain genes—the sections of DNA that control cell growth and death—cells can start to behave abnormally.

This damage does not happen all at once. Most cancers develop over many years as cells accumulate more and more genetic mistakes. A single mutation usually is not enough to cause cancer. Instead, a cell needs multiple mutations in different genes before it becomes truly cancerous. This is why cancer is more common as people age—they have had more time for damage to build up.

Some people inherit a genetic mutation from a parent that puts them at higher risk for certain cancers. These inherited mutations are rare but powerful. For example, mutations in the BRCA1 or BRCA2 genes significantly increase the risk of breast and ovarian cancer. Most cancers, though, are not inherited. They develop from mutations that happen during a person's lifetime due to exposure to risk factors.

Tobacco, alcohol, and sun exposure as major causes

Tobacco is the single largest preventable cause of cancer worldwide. Smoking damages the cells lining the lungs and throat. Chewing tobacco and secondhand smoke also increase cancer risk. Tobacco contains over 70 chemicals known to damage DNA.

Alcohol increases the risk of several cancers, including breast, liver, colon, and throat cancer. The risk rises with how much and how often a person drinks. Alcohol can damage cells directly and also interferes with how the body repairs DNA damage.

Sun exposure is the main cause of skin cancer. Ultraviolet (UV) radiation from the sun damages the DNA in skin cells. The risk is higher for people with fair skin and for those who get severe sunburns, especially in childhood. Tanning beds emit UV radiation and carry the same risk as sun exposure.

Obesity, diet, and physical activity

Being overweight or obese increases the risk of several cancers, including breast cancer (in postmenopausal women), colon cancer, endometrial cancer, and pancreatic cancer. Extra body fat produces hormones and proteins that can promote cell growth and inflammation, both of which can lead to cancer.

Diet plays a role in cancer risk. Eating large amounts of red meat and processed meat is linked to colon and rectal cancer. A diet high in vegetables, fruits, and whole grains may lower cancer risk, though the exact protective effect is still being studied. Processed foods high in salt and sugar are associated with higher cancer rates.

Physical inactivity contributes to cancer risk partly through weight gain and partly through its effects on hormones and inflammation. Regular physical activity may lower the risk of several cancers.

Infections and viruses that can lead to cancer

Certain infections increase cancer risk by causing long-term inflammation or by directly damaging cells. Human papillomavirus (HPV) causes most cervical cancers and some cancers of the throat, anus, and genitals. Hepatitis B and C viruses can lead to liver cancer after years of infection. Human T-cell leukemia virus (HTLV-1) causes a rare type of leukemia.

Helicobacter pylori is a bacterium that infects the stomach and increases the risk of stomach cancer and a type of lymphoma. Epstein-Barr virus is linked to certain lymphomas and nasopharyngeal cancer, particularly in people from certain geographic regions.

Vaccines now prevent some of these infections. The HPV vaccine prevents the strains of HPV that cause most cervical cancers. The hepatitis B vaccine prevents hepatitis B infection. These vaccines reduce cancer risk when given before infection occurs.

Hormones and reproductive factors

Hormones influence cancer risk, particularly for breast, ovarian, and endometrial cancers. Estrogen and progesterone affect how breast cells grow and divide. Women who have more lifetime exposure to estrogen—through early menstruation, late menopause, or hormone replacement therapy—have a higher breast cancer risk.

Pregnancy and breastfeeding lower breast cancer risk. The more pregnancies a woman has had and the longer she breastfeeds, the lower her risk. Oral contraceptives slightly increase breast cancer risk while a woman is taking them, but the risk returns to normal after she stops.

For men, testosterone levels influence prostate cancer risk. Men with higher testosterone levels have a higher risk, though the relationship is complex and not fully understood.

Environmental exposures and occupational hazards

Long-term exposure to certain chemicals increases cancer risk. Asbestos, once widely used in insulation and building materials, causes mesothelioma and lung cancer. Radon, a radioactive gas that seeps into homes from soil, is the second leading cause of lung cancer after smoking. Benzene, used in manufacturing, increases leukemia risk. Formaldehyde, found in some building materials and products, is linked to nasopharyngeal cancer and leukemia.

People in certain occupations face higher exposure to cancer-causing substances. Workers in agriculture may be exposed to pesticides. Those in construction may encounter asbestos. Factory workers may be exposed to various chemicals. Occupational safety regulations exist to limit these exposures, but risk remains for workers in some industries.

Air pollution and secondhand smoke also contribute to cancer risk. Living in areas with high air pollution increases lung cancer risk even in people who do not smoke.

Age, family history, and other personal factors

Age is one of the strongest risk factors for cancer. Most cancers occur in people over 65. As people age, cells have had more time to accumulate genetic damage, and the body's ability to repair DNA damage and fight abnormal cells declines.

Family history matters when multiple relatives have had cancer, especially if they were diagnosed at a young age or had the same type of cancer. This pattern suggests a shared genetic mutation or shared environmental exposures. However, having a family member with cancer does not mean you will develop it.

Race and ethnicity affect cancer risk for some types. For example, African American men have higher prostate cancer rates and worse outcomes than white men. These differences reflect a combination of genetic factors, differences in screening and access to care, and socioeconomic factors. Immigrant status and where someone lives also influence cancer risk through differences in exposure, screening, and healthcare access.

Frequently Asked Questions

Can you get cancer from stress?

Stress does not directly cause cancer, but chronic stress may increase cancer risk indirectly by weakening the immune system, promoting inflammation, and leading to unhealthy behaviors like smoking or poor diet. Managing stress through exercise, sleep, and social connection is part of overall health.

Does cancer run in families?

Some cancers do run in families when a genetic mutation is inherited. However, most cancers are not inherited. Having a parent or sibling with cancer increases your risk, but it does not mean you will develop cancer. Genetic testing can identify inherited mutations in some families.

If I have a risk factor, will I definitely get cancer?

No. Having a risk factor means your chances are higher than average, but many people with risk factors never develop cancer. Similarly, some people without known risk factors do develop cancer. Risk factors are probabilities, not certainties.

Can cancer be prevented?

You cannot prevent cancer entirely, but you can reduce your risk. Not smoking, limiting alcohol, protecting your skin from sun, maintaining a healthy weight, eating a diet rich in vegetables and fruits, and staying physically active all lower cancer risk. Vaccines against HPV and hepatitis B prevent certain cancers.

Is cancer always fatal?

No. Many cancers are treatable, especially when caught early. Survival rates vary widely depending on the type of cancer, how advanced it is when found, and the person's age and overall health. Some cancers are chronic conditions that people live with for years.