Cells are the very small units that make up all living things, including the human body. Cell growth and cell division are essential events that are involved in natural process of replacing worn-out or dying cells, and healing of wounds. Normal body cells grow, divide, and die in an orderly fashion. These normal growth, cell division and cell death are strictly controlled and regulated by the body mechanisms. Many control mechanisms exists for the control of cell growth and cell division. Genes that are located in the nucleus of the cells are the major controlling forces in the cells. This control is usually exerted through many genes that work in a co-coordinated manner. Cancer develops when these growth mechanisms are disrupted. When this happens cells begin to grow neglecting all growth control mechanisms.

resulting in cancer. Cancer is not a single disease, but is a group of many related diseases that all behave in the same way in terms of cell growth and neglecting body control mechanisms for growth. Because the body does not have control over the growth of the cancer cells; they grow, invade, erode and destroy surrounding normal tissue. The human body is made up of hundreds of different kinds of cells; all of them behave differently from each other at least in some respects. There are different sets of genes for growth promotion and growth suppression. In normal healthy state the genes that promote growth, and genes that suppresses the growth are in good harmony, so that the cell growth and cell division occur only when it is needed. These growth related genes can get damaged by internal or external factors. If a growth suppressing gene is damaged the control mechanism suppressing unwanted growth may be lost. If a damage occurs in the growth promoting genes, they may lose their ability to promote cell growth, but some types of damage can actually increase the growth promoting ability of these types of genes. This may happen because the part of the gene that senses signals from the growth suppressing genes may be damaged releasing it from the control mechanisms of the growth suppressing genes. As the result of this type of damage, the genes the cell no longer pays attention to the commands of the tumor suppressor genes and continue to multiply. Every cell is programmed to die after certain number of cell divisions. Some times damage can occur to this programmed cell death gene that controls the death of the cell, thereby giving the cell the ability of immortality. If a combination ability to divide without control mechanisms and immortality are achieved that will give the cell very high chance of developing in to cancer. Most of the time only one cell accumulates all the genetic changes that are required to develop a cancer, but it soon will divide to multiple cells. Every time these abnormal cancer cells divide, the traits of lack of control by body growth mechanisms and immortality are passed to each of the daughter cells thereby, perpetuating the process of uncontrolled cell growth.

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Cancer is a class of diseases or disorders characterized by uncontrolled division of cells and the ability of these cells to invade other tissues, either by direct growth into adjacent tissue through invasion or by implantation into distant sites by metastasis (in which cancer cells are transported through the blood or lymphatic system). Cancer may affect people at all ages, but risk increases with age. It is one of the leading causes of death in developed countries.

There are many types of cancer. Severity of symptoms depends on the site and character of the malignancy and whether there is metastasis. A definitive diagnosis usually requires the histologic examination of tissue by a pathologist. This tissue is obtained by biopsy or surgery. Most cancers can be treated and some cured, depending on the specific type, location, and stage. Once diagnosed, cancer is usually treated with a combination of surgery, chemotherapy and radiotherapy. As research develops, treatments are becoming more specific for the type of cancer pathology. Drugs that target specific cancers already exist for many cancers. If untreated, cancers may eventually cause illness and death, though this is not always the case.

The unregulated growth that characterizes cancer is caused by damage to DNA, resulting in mutations to genes that encode for proteins controlling cell division. Many mutation events may be required to transform a normal cell into a malignant cell. These mutations can be caused by chemicals or physical agents called carcinogens, by close exposure to radioactive materials, or by certain viruses that can insert their DNA into the human genome. Mutations occur spontaneously, or are passed down generations as a result of germ line mutations.

Many forms of cancer are associated with exposure to environmental factors such as tobacco smoke, radiation, alcohol and certain viruses. While some of these can be avoided, there is no known way to entirely avoid the disease, and public health and vaccination programs are also important on a global scale.