Cancer (medical term: malignant neoplasm) is a class of diseases in which a cell, or a group of cells display uncontrolled growth through division beyond the normal limits, invasion that intrudes upon and destroys adjacent tissues, and sometimes metastasis, which spreads the cells to other locations in the body via lymph or blood. These three malignant properties of cancers differentiate them from benign tumors, which are self-limited, and do not invade or metastasize. Most cancers form a tumor but some, like leukemia, do not. The branch of medicine concerned with the study, diagnosis, treatment, and prevention of cancer is oncology.

Cancer can affect people at all ages with the risk for most types increasing with age. It caused about 13% of all human deaths in 2007 (7.6 million). Cancers are primarily an environmental disease with 90-95% of cases due to lifestyle and environmental factors and 5-10% due to genetics. Common environmental factors leading to cancer death include: tobacco (25-30%), diet and obesity (30-35%), infections (15-20%), radiation, stress, lack of physical activity, and environmental pollutants. These environmental factors cause abnormalities in the genetic material of cells.