Cancer is a disease in which abnormal cells divide without control.  These cells can create their own growth signals, avoid apoptosis and immune destruction, invade neighbouring tissues, undergo metastasis and angiogenesis, and alter energy metabolism.

For a normal cell to transform into a cancer cell, multiple gene changes are required.  Initial errors compound into more severe errors, progressively allowing the cell to escape controls that would limit normal tissue growth.   Cancer genes are divided into oncogenes that promote cell growth, and suppressor genes that inhibit cell division and survival. Malignant transformation can occur through the formation of novel oncogenes, the over-expression of normal oncogenes, or the disabling of suppressor genes.

There are more than 100 different types of human cancer.  The main categories are carcinoma - cancers derived from epithelial cells, sarcoma - cancers arising from connective tissue, lymphoma and leukaemia, which arise from hematopoietic cells and blastoma - cancers derived from immature precursor cells.  Most human cancers (90–95% of cases) are due to genetic mutations from environmental factors, with the remainder caused by inherited genetic defects.  Environmental factors that contribute to cancer in humans are smoking (25–30%), diet and obesity (30–35%), infection (15–20%), radiation (up to 10%), and lack of physical activity.