Definition:
 Cancer is not a single disease, but encompasses a large group of almost 100 diseases that have certain symptoms in common. The two main characteristics of all cancers are uncontrolled growth of abnormal cells that form masses, or tumors, in the body and the ability of these cells to migrate from the original site, invade other tissues at distant sites, and form new tumors. This process is called metastasis. If metastasis not controlled, cancer can result in death.

 Description:
 Each year, over one million Americans are newly diagnosed with some form of cancer. About one in every four deaths (about 554,000 people in 2007) in the United States is from cancer. Cancer is second only to heart disease as a cause of death. People of any age, race, and gender can get cancer, although more than three-quarters of cancers are diagnosed in people over age 55. The most common cancers are non-melanoma skin cancer, lung cancer, colon cancer, breast cancer (in women), and prostate cancer (in men).
  In addition, cancer of the kidneys, ovaries, uterus, pancreas, bladder, rectum, and blood and lymph node cancer (leukemias and lymphomas) are also included among the cancers that most often affect Americans. Although all racial groups are affected by cancer, African American men get cancer at a higher rate than any other racial groups, followed in number by white men, then African American women. Cancer is thought to occur because of small changes (mutations) in genes. A gene is a small packet of deoxyribonucleic acid(DNA), the master molecule of the cell that is inherited from each parent.
  Genes control all aspects of development and metabolism. Small changes in the structure of genes can cause changes in proteins that regulate body functions. Throughout life, most cells in the body grow, multiply, and replace themselves. During the multiplication process, errors or changes, called mutations, occur. Some mutations are harmless, but others cause the production of altered or faulty proteins. In normal cells, growth and development is strictly controlled. For example, when a wound heals, cells that replace the lost skin stop growing when the wound site is covered. When certain mutations occur, some cells become abnormal and continue to grow and divide long after a normal cell would have stopped. When this happens, the cell is said to be malignant or cancerous.
  Malignant cells look different from normal cells under the microscope. Eventually the malignant cell divides often enough to form a mass called a tumor or neoplasm (new growth). Abnormal cells are formed all the time from mutations, but in a healthy individual, the immune system can recognizes these cells as abnormal and destroy them. However, some mutant cells may escape destruction and survive to grow into tumors. There are two types of tumors, benign or malignant. A benign tumor is not malignant and is not considered cancer. Benign tumors are slow growing, do not spread or invade surrounding tissue, and once removed, often do not recur. A malignant tumor, on the other hand, is cancerous. It often grows rapidly (although there are slow-growing cancers) and invades surrounding tissue.
  Cells break off from the tumor and move through circulatory system to other organs in the body where they settle, continue to divide, and establish new tumors. Even when the original, or primary, tumor is removed, if the cancer cells have spread or if not all the primary cells are removed or killed, cancer will reoccur. Researchers believe that cancers are caused by multiple factors.
  About three-quarters of all cancers are thought to be caused by changes in the cell's DNA that occur because of interaction with the environment. Environmental and lifestyle risk factors include tobacco use (the cause of one-third of all cancers), exposure to radiation (skin cancer) and exposure to workplace pollutants such as asbestos. Some cancers are caused by faulty DNA in genes inherited from the individual's parents, However, fewer than 10% of all cancers are clearly hereditary. The most common causes of cancer appear to involve an interaction between the individual's genetics, the environment, and lifestyle choices. In other words, an individual's inherited genes may not directly cause cancer, but may make the individual more likely to develop cancer if exposed to certain environmental triggers. Lifestyle choices, such as diet, drug, alcohol, and tobacco use, also tend to play a role in triggering cancer. Some people are at higher risk for cancer than others, but many people with multiple risk factors (e.g. family history of cancer, tobacco use, exposure to chemicals) do not develop cancer, while some people with no obvious risk factors do.

Cancer describes many specific diseases and not all cancers are the same:

• Carcinomas are cancers that arise in the epithelium (the layer of cells covering the body's surface and lining the internal organs and various glands). Ninety percent of human cancers fall into this category. Carcinomas can be subdivided into two types: adenocarcinomas and squamous cell carcinomas. Adenocarcinomas are cancers that develop in an organ or a gland. Squamous cell carcinomas are cancers that originate in the skin.

• Melanomas also originate in the skin, usually in the pigment cells (melanocytes).

• Sarcomas are cancers of the supporting tissues of the body, such as bone, muscle, and blood vessels.

• Cancers of the blood and lymph glands are called leukemias and lymphomas respectively.

• Gliomas are cancers of the nerve tissue.

Frequency Of Cancer-Related Death
Cancer Site Number of Deaths Per Year Lung 160,100 Colon and rectum 56,500 Breast 43,900 Prostate 39,200 Pancreas 28,900 Lymphoma 26,300 Leukemia 21,600 Brain 17,400 Stomach 13,700 Liver 13,000 Esophagus 11,900 Bladder 12,500 Kidney 11,600 Multiple myeloma 11,300

In many cases the cause (or causes) of cancer is unknown. Most experts believe that cancers have multiple causes. The major risk factors for cancer are tobacco use, alcohol use, poor diet, unsafe sexual and reproductive behavior, infectious agents, family history, and exposure to workplace or environmental pollution.

Lung cancer is the leading cause of cancer death in the United States. Most people diagnosed with lung cancer are over age 45 and die within two years. More 80% of lung cancer cases occur in smokers. Smoking has also been shown to be a contributory factor in cancers of upper respiratory tract (the airways), esophagus, larynx, bladder, pancreas, and possibly liver, stomach, breast, and kidney as well. Recently, scientists have also shown that second-hand smoke (passive smoking) can increase one's risk of developing cancer.

Excessive consumption of alcohol is a risk factor in certain cancers, such as liver cancer. Alcohol, in combination with tobacco, significantly increases the chances that an individual will develop mouth, pharynx, larynx, and esophageal cancers.

Researchers estimate that poor diet is a risk factor in as many as one-third of all cancers. A diet high in fats, especially combined with physical inactivity and obesity, is considered a risk factor in cancers of the breast, colon, rectum, pancreas, prostate, gall bladder, ovaries, and uterus.

The human papillomavirus, which is sexually transmitted, has been shown to cause cancer of the cervix. Having multiple sex partners and becoming sexually active early increases a woman's chance of contracting cervical cancer. In 2007, a vaccine against the virus that causes cervical cancer was introduced. The vaccination process requires three doses of vaccine spread over one year and is paid for by some insurance companies. Other studies have linked a higher risk of breast and ovarian cancer to women who do not have children or who have their first child after age 20.

Research scientists have come to believe that about 15% of the world's cancer deaths can be traced to viruses, bacteria, or parasites. The most common cancer-causing pathogens and the cancers associated with them are shown in table form below.

Certain cancers like breast, colon, ovarian, and uterine cancer tend to run in families. A few cancers, such as the eye cancer "retinoblastoma," a subtype of colon cancer, and a subtype of breast cancer known as "early-onset breast cancer," have been definitively shown to be linked to certain genes that can be tracked within a family. It appears that inheriting specific genes makes a person susceptible to certain types of cancers.

Certain occupations expose workers to hazards that increase the risk of cancer. For example, asbestos workers have an increased incidence of a specific type of lung cancer. Similarly, a higher likelihood of getting bladder cancer is associated with dye, rubber, and gas workers; skin and lung cancer with smelters, gold miners and arsenic workers; leukemia with glue and varnish workers; liver cancer with PVC manufacturers; and lung, bone and bone marrow cancer with radiologists and uranium miners.

Exposure to radiation increases the risk of skin cancer. High doses of ultra-violet radiation from exposure to the sun without using sunblock protection accounts for a majority of melanoma deaths. Other sources of radiation are x rays, radon gas, and ionizing radiation from nuclear material.

Papillomaviruses Cancer of the cervix
Hepatitis B virus Liver cancer
Hepatitis C virus Liver cancer
Epstein-Barr virus Burkitt's lymphoma Cancers of the upper pharynx
Hodgkin's lymphoma, Non-Hodgkin's lymphoma, Gastric cancers Human immunodeficiency virus (HIV) Kaposi's sarcoma Lymphoma Bacteria
Helicobacter pylori Stomach cancer Lymphomas

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