Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body.[2][8] These contrast with benign tumors, which do not spread to other parts of the body.[8] Possible signs and symptoms include a lump, abnormal bleeding, prolonged cough, unexplained weight loss, and a change in bowel movements.[1] While these symptoms may indicate cancer, they may have other causes.[1] Over 100 types of cancers affect humans.[8]

Tobacco use is the cause of about 22% of cancer deaths.[2] Another 10% are due to obesity, poor diet, lack of physical activity, and excessive drinking of alcohol.[2][9][10] Other factors include certain infections, exposure to ionizing radiation and environmental pollutants.[3] In the developing world nearly 20% of cancers are due to infections such as hepatitis B, hepatitis C and human papillomavirus infection.[2] These factors act, at least partly, by changing the genes of a cell.[11] Typically many genetic changes are required before cancer develops.[11] Approximately 5–10% of cancers are due to inherited genetic defects from a person's parents.[12] Cancer can be detected by certain signs and symptoms or screening tests.[2] It is then typically further investigated by medical imaging and confirmed by biopsy.[13]

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.[14][15] Early detection through screening is useful for cervical and colorectal cancer.[16] The benefits of screening in breast cancer are controversial.[16][17] Cancer is often treated with some combination of radiation therapy, surgery, chemotherapy, and targeted therapy.[2][4] Pain and symptom management are an important part of care.[2] Palliative care is particularly important in people with advanced disease.[2] The chance of survival depends on the type of cancer and extent of disease at the start of treatment.[11] In children under 15 at diagnosis the five-year survival rate in the developed world is on average 80%.[18] For cancer in the United States the average five-year survival rate is 66%.[5]

In 2015, about 90.5 million people had cancer.[6] About 14.1 million new cases occur a year (not including skin cancer other than melanoma).[11] It caused about 8.8 million deaths (15.7% of deaths).[7] The most common types of cancer in males are lung cancer, prostate cancer, colorectal cancer and stomach cancer.[19] In females, the most common types are breast cancer, colorectal cancer, lung cancer and cervical cancer.[11] If skin cancer other than melanoma were included in total new cancers each year, it would account for around 40% of cases.[20][21] In children, acute lymphoblastic leukemia and brain tumors are most common except in Africa where non-Hodgkin lymphoma occurs more often.[18] In 2012, about 165,000 children under 15 years of age were diagnosed with cancer.[19] The risk of cancer increases significantly with age and many cancers occur more commonly in developed countries.[11] Rates are increasing as more people live to an old age and as lifestyle changes occur in the developing world.[22] The financial costs of cancer were estimated at $1.16 trillion USD per year as of 2010.[23]

Cancers are a large family of diseases that involve abnormal cell growth with the potential to invade or spread to other parts of the body.[2][8] They form a subset of neoplasms. A neoplasm or tumor is a group of cells that have undergone unregulated growth and will often form a mass or lump, but may be distributed diffusely.[24][25]

All tumor cells show the six hallmarks of cancer. These characteristics are required to produce a malignant tumor. They include:[26]

Cell growth and division absent the proper signals
Continuous growth and division even given contrary signals
Avoidance of programmed cell death
Limitless number of cell divisions
Promoting blood vessel construction
Invasion of tissue and formation of metastases[27]
The progression from normal cells to cells that can form a detectable mass to outright cancer involves multiple steps known as malignant progression.[27][28]

When cancer begins, it produces no symptoms. Signs and symptoms appear as the mass grows or ulcerates. The findings that result depend on the cancer's type and location. Few symptoms are specific. Many frequently occur in individuals who have other conditions. Cancer is a "great imitator". Thus, it is common for people diagnosed with cancer to have been treated for other diseases, which were hypothesized to be causing their symptoms.[29]

People may become anxious or depressed post-diagnosis. The risk of suicide in people with cancer is approximately double.[30]

Local symptoms
Local symptoms may occur due to the mass of the tumor or its ulceration. For example, mass effects from lung cancer can block the bronchus resulting in cough or pneumonia; esophageal cancer can cause narrowing of the esophagus, making it difficult or painful to swallow; and colorectal cancer may lead to narrowing or blockages in the bowel, affecting bowel habits. Masses in breasts or testicles may produce observable lumps. Ulceration can cause bleeding that, if it occurs in the lung, will lead to coughing up blood, in the bowels to anemia or rectal bleeding, in the bladder to blood in the urine and in the uterus to vaginal bleeding. Although localized pain may occur in advanced cancer, the initial swelling is usually painless. Some cancers can cause a buildup of fluid within the chest or abdomen.[29]

Systemic symptoms
General symptoms occur due to effects that are not related to direct or metastatic spread. These may include: unintentional weight loss, fever, excessive fatigue and changes to the skin.[31] Hodgkin disease, leukemias and cancers of the liver or kidney can cause a persistent fever.[29]

Some cancers may cause specific groups of systemic symptoms, termed paraneoplastic syndrome. Examples include the appearance of myasthenia gravis in thymoma and clubbing in lung cancer.[29]

Metastasis
Main article: Metastasis
Cancer can spread from its original site by local spread, lymphatic spread to regional lymph nodes or by hematogenous spread via the blood to distant sites, known as metastasis. When cancer spreads by a hematogenous route, it usually spreads all over the body. However, cancer 'seeds' grow in certain selected site only ('soil') as hypothesized in the soil and seed hypothesis of cancer metastasis. The symptoms of metastatic cancers depend on the tumor location and can include enlarged lymph nodes (which can be felt or sometimes seen under the skin and are typically hard), enlarged liver or enlarged spleen, which can be felt in the abdomen, pain or fracture of affected bones and neurological symptoms.[29]

The majority of cancers, some 90–95% of cases, are due to genetic mutations from environmental factors.[3] The remaining 5–10% are due to inherited genetics.[3] Environmental, as used by cancer researchers, means any cause that is not inherited genetically, such as lifestyle, economic and behavioral factors and not merely pollution.[32] Common environmental factors that contribute to cancer death include tobacco (25–30%), diet and obesity (30–35%), infections (15–20%), radiation (both ionizing and non-ionizing, up to 10%), stress, lack of physical activity and pollution.[3][33]

It is not generally possible to prove what caused a particular cancer because the various causes do not have specific fingerprints. For example, if a person who uses tobacco heavily develops lung cancer, then it was probably caused by the tobacco use, but since everyone has a small chance of developing lung cancer as a result of air pollution or radiation, the cancer may have developed for