What is cancer?

 Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. While these symptoms may indicate cancer, they can also have other causes. Another 10% are due to obesity, poor diet, lack of physical activity, or excessive drinking of alcohol. Other factors include certain infections, exposure to ionizing radiation, and environmental pollutants. In the developing world, 15% of cancers are due to infections such as Helicobacter pylori, hepatitis B, hepatitis C, human papillomavirus infection, Epstein–Barr virus, and human immunodeficiency virus. Typically, many genetic changes are required before cancer develops. Cancer can be detected by certain signs and symptoms or screening tests. The risk of developing certain cancers can be reduced by not smoking, maintaining a healthy weight, limiting alcohol intake, eating plenty of vegetables, fruits, and whole grains, vaccination against certain infectious diseases, limiting consumption of processed meat and red meat, and limiting exposure to direct sunlight. Early detection through screening is useful for cervical and colorectal cancer. The benefits of screening for breast cancer are controversial. Cancer is often treated with some combination of radiation therapy, surgery, chemotherapy, and targeted therapy. Pain and symptom management are an important part of care. For cancer in the United States, the average five-year survival rate is 66%. In 2015, about 90.5 million people worldwide had cancer. In 2019, annual cancer cases grew to 23.6 million people and 10 million deaths worldwide, representing over the previous decade increases of 26% and 21%, respectively. 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. In children, acute lymphoblastic leukemia and brain tumors are most common, except in Africa, where non-Hodgkin lymphoma occurs more often. The financial costs of cancer were estimated at 1.16 trillion USD per year. Etymology and definitions The word comes from the ancient Greek καρκίνος, meaning crab and tumor. Greek physicians Hippocrates and Galen, among others, noted the similarity of crabs to some tumors with swollen veins. The word was introduced in English in the modern medical sense around 1600. Cancers comprise a large family of diseases that involve abnormal cell growth with the potential to invade or spread to other parts of the body. All tumor cells show the six hallmarks of cancer. These characteristics are required to produce a malignant tumor. They include Signs and symptoms When cancer begins, it produces no symptoms. Signs and symptoms appear as the mass grows or ulcerates. Local symptoms may occur due to the mass of the tumor or its ulceration. Systemic symptoms may occur due to the body's response to cancer. This may include fatigue, unintentional weight loss, or skin changes. Some cancers can cause a systemic inflammatory state that leads to ongoing muscle loss and weakness, known as cachexia. Some cancers, such as Hodgkin's disease, leukemia, and liver or kidney cancers, can cause a persistent fever. Metastasis is the spread of cancer to other locations in the body. The dispersed tumors are called metastatic tumors, while the original is called the primary tumor. Almost all cancers can metastasize. Metastasis is common in the late stages of cancer and it can occur via the blood or the lymphatic system or both. The typical steps in metastasis are local invasion, intravasation into the blood or lymph, circulation through the body, extravasation into the new tissue, proliferation,n, and angiogenesis. Different types of cancers tend to metastasize to particular organs, but overall the most common places for metastases to occur are the lungs, liver, brain,n, and bones. Causes The majority of cancers, some 90–95% of cases, are due to genetic mutations from environmental and lifestyle factors. Common environmental factors that contribute to cancer death include tobacco use, diet and obesity, infections, radiation, lack of physical activity, and pollution. Psychological stress does not appear to be a risk factor for the onset of cancer, though it may worsen outcomes in those who already have cancer. however factors that may have contributed to the development of cancer can be transmissible; such as oncoviruses like hepatitis B, Epstein-Barr virus,s, and HIV. Chemicals Exposure to particular substances has been linked to specific types of cancer. These substances are called carcinogens. Tobacco smoke, for example, causes 90% of lung cancer. It also causes cancer in the larynx, head, neck, stomach, bladder, kidney, esophagus,s and pancreas. Tobacco smoke contains over fifty known carcinogens, including nitrosamines and polycyclic aromatic hydrocarbons. and about one in three in the developed world. Lung cancer death rates in the United States have mirrored smoking patterns, with increases in smoking followed by dramatic increases in lung cancer death rates and, more recently, decreases in smoking rates since the 1950s followed by decreases in lung cancer death rates in men since 1990. In Western Europe, 10% of cancers in males and 3% of cancers in females are attributed to alcohol exposure, especially liver and digestive tract cancers. Cancer from work-related substance exposures may cause between 2 and 20% of cases, causing at least 200,000 deaths. Cancers such as lung cancer and mesothelioma can come from inhaling tobacco smoke or asbestos fibers, or leukemia from exposure to benzene. Diet and exercise Diet, physical inactivity,y, and obesity are related to up to 30–35% of cancer deaths. In the United States, excess body weight is associated with the development of many types of cancer and is a factor in 14–20% of cancer deaths. Physical inactivity is believed to contribute to cancer risk, not only through its effect on body weight but also through negative effects on the immune system and endocrine system. National differences in dietary practices may partly explain differences in cancer incidence. For example, gastric cancer is more common in Japan due to its high-salt diet while colon cancer is more common in the United States. Immigrant cancer profiles mirror those of their new country, often within one generation. Infection Worldwide approximately 18% of cancer deaths are related to infectious diseases. Parasitic infections associated with cancer include Schistosoma haematobium and liver flukes, Opisthorchis viverrine, I, and Clonorchis Sinensis. Radiation exposure such as ultraviolet radiation and radioactive material is a risk factor for cancer. Many non-melanoma skin cancers are due to ultraviolet radiation, mostly from sunlight. Residential exposure to radon gas, for example, has similar cancer risks as passive smoking. Prolonged exposure to ultraviolet radiation from the sun can lead to melanoma and other skin malignancies. Clear evidence establishes ultraviolet radiation, especially the non-ionizing medium wave UVB, as the cause of most non-melanoma skin cancers, which are the most common forms of cancer in the world. Evidence, however, has not supported a concern. Heredity The vast majority of cancers are non-hereditary. Hereditary cancers are primarily caused by an inherited genetic defect. Less than 0.3% of the population are carriers of a genetic mutation that has a large effect on cancer risk and these cause less than 3–10% of cancer. Some of these syndromes include certain inherited mutations in the genes BRCA1 and BRCA2 with a more than 75% risk of breast cancer and ovarian cancer, among others.