Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body.[1][2] Not all tumors are cancerous; benign tumors do not spread to other parts of the body.[2] Possible signs and symptoms include a lump, abnormal bleeding, prolonged cough, unexplained weight loss and a change in bowel movements.[3] While these symptoms may indicate cancer, they may have other causes.[3] Over 100 types of cancers affect humans.[2]

Tobacco use is the cause of about 22% of cancer deaths.[1] Another 10% is due to obesity, poor diet, lack of physical activity and drinking alcohol.[1][4] Other factors include certain infections, exposure to ionizing radiation and environmental pollutants.[5] In the developing world nearly 20% of cancers are due to infections such as hepatitis B, hepatitis C and human papillomavirus (HPV).[1] These factors act, at least partly, by changing the genes of a cell.[6] Typically many genetic changes are required before cancer develops.[6] Approximately 5–10% of cancers are due to inherited genetic defects from a person's parents.[7] Cancer can be detected by certain signs and symptoms or screening tests.[1] It is then typically further investigated by medical imaging and confirmed by biopsy.[8]

Many cancers can be prevented by not smoking, maintaining a healthy weight, not drinking too much alcohol, eating plenty of vegetables, fruits and whole grains, vaccination against certain infectious diseases, not eating too much processed and red meat, and avoiding too much sunlight exposure.[9][10] Early detection through screening is useful for cervical and colorectal cancer.[11] The benefits of screening in breast cancer are controversial.[11][12] Cancer is often treated with some combination of radiation therapy, surgery, chemotherapy, and targeted therapy.[1][13] Pain and symptom management are an important part of care. Palliative care is particularly important in people with advanced disease.[1] The chance of survival depends on the type of cancer and extent of disease at the start of treatment.[6] In children under 15 at diagnosis the five-year survival rate in the developed world is on average 80%.[14] For cancer in the United States the average five-year survival rate is 66%.[15]

In 2012 about 14.1 million new cases of cancer occurred globally (not including skin cancer other than melanoma).[6] It caused about 8.2 million deaths or 14.6% of human deaths.[6][16] The most common types of cancer in males are lung cancer, prostate cancer, colorectal cancer and stomach cancer. In females, the most common types are breast cancer, colorectal cancer, lung cancer and cervical cancer.[6] If skin cancer other than melanoma were included in total new cancers each year it would account for around 40% of cases.[17][18] In children, acute lymphoblastic leukaemia and brain tumors are most common except in Africa where non-Hodgkin lymphoma occurs more often.[14] In 2012, about 165,000 children under 15 years of age were diagnosed with cancer. The risk of cancer increases significantly with age and many cancers occur more commonly in developed countries.[6] Rates are increasing as more people live to an old age and as lifestyle changes occur in the developing world.[19] The financial costs of cancer were estimated at $1.16 trillion US dollars per year as of 2010.