Causes of cancer

Cancer arises when genetic changes cause cells to grow uncontrollably and form tumors. In most cases—known as sporadic cancers—the root cause is DNA damage and genomic instability. Only a small number of cancers stem from inherited genetic mutations. The majority are linked to exposures from the environment, lifestyle, or behavior. Cancer is generally not contagious in humans, though certain oncoviruses and cancer bacteria can trigger it. When researchers talk about "environmental" causes, they mean everything outside the body that interacts with us—not just the physical environment like air pollution or sunlight, but also lifestyle and behavioral choices.

Worldwide, over one-third of cancer deaths (and about 75–80% in the United States) could potentially be avoided by reducing exposure to known risk factors. Common contributors include tobacco use (responsible for 25–30% of cancer deaths), diet and obesity (20–25% of cancer cases, with a lower percentage for deaths), infections (15–20%), radiation—both ionizing and non-ionizing (up to 10%), and environmental pollutants. These factors work, at least in part, by altering gene function inside cells. Typically, many such genetic changes must accumulate before cancer develops. Aging is also a key factor, as molecular and cellular changes that drive cancer build up over time and eventually lead to the disease.

**Genetics**

Though more than 50 hereditary forms of cancer exist, about 5–10% of people carry a cancer-related inherited genetic mutation, and these account for less than 3–10% of all cancer cases. The vast majority are non-hereditary, or sporadic. Hereditary cancers are mainly caused by an inherited genetic defect. A cancer syndrome—or family cancer syndrome—is a genetic disorder where inherited mutations in one or more genes make individuals more prone to developing cancer, often at an earlier age. The risk varies by syndrome; for some, cancer is not the primary feature and occurs only rarely. Many cancer syndrome cases involve mutations in tumor suppressor genes that normally regulate cell growth. Other common mutations affect DNA repair genes, oncogenes, or genes involved in blood vessel formation. For example, inherited mutations in BRCA1 and BRCA2 carry a more than 75% risk of breast and ovarian cancer.