Cancer caused about 25% of deaths in Britain in 1991 and is the most common cause of death after cardiovascular disease. This is typical of developed countries. Breast cancer is the most common cancer is not a single disease more than 200 types of cancer are known. Cancer is a result of uncontrolled cell division. The type of nuclear divisions. The problem is caused by mutation or abnormal activation of the genes which control cell division. When the genes are abnormal they are called oncogene. About 100 of these have been discovered. A single faulty cell may divide to form a clone of identical cells. Eventually an tumour is formed. Tumour cells can break away and spread to other parts of the body, particularlyin the tumours or metastases. This process is called metastasis. Tumours that spread and eventually cause ill health and death are described as malignant. The majority of tumours, such as common warts, does not spread and are described as benign.

Changes in the genes are called mutations and any factor bringing about a mutation is called a mutagen. An agent which causes cancer is called a carcinogen. Most mutated cells are either destroyed by the body’s immune system or die with no ill effect on the body. It is believed that development of a malignant cancer cell involves several steps and is usually caused by more that one factor operating over several years rather than a single factor. More than one mutation to the genes may occur. Up to 20% of the cancer worldwide may be caused by viruses.

Retro viruses: Evidence that cancers are genetic in origin was provided by work with retroviruses. Retroviruses are RNA viruses which, when they invade animal cells, use the enzyme reverse transcriptase to make DNA copies of the viral RNA. The DNA is inserted into the host DNA where it may stay and be replicated for generations of cells. Some retroviruses are harmless. However, HIV is a harmful retrovirus and other retrovirus cause cancer. These contain a gene which alters retroviruses cause cancer. These contain a gene which host cell divisions genes, switching them on and causing the cell to become malignant. The genes become oncogenes. The advantage to the virus is that the cell makes many copies of itself and therefore of the virus.

DNA viruses: DNA viruses contain DNA as their hereditary material. Some contain their own oncogene which can cause uncontrolled cell division of host cells. Examples which infect humans are the papilloma viruses have been implicated in some forms of cervical cancer, making this a sexually transmitted disease. The Epstein-Barr viruses may cause one form of Burkitt’s lymphoma which is common in Africa.

Hereditary predisposition: About 5% of human cancer shows a strong genetic predisposition, in other word they tend to run in families. More than 40 types of cancer including cancer of the breast, ovary and colon, come into this category. The genes responsible may be oncogenes, or genes which lead to failure to kill cancer cells. In most cases other factors are required but in a few case such as retinoblastoma, a single faulty gene is responsible. Retinoblastoma starts in the eye and spreads to the brain causing death if untreated. It is caused by a dominant gene. Two breast cancer genes have been identified and named BRCA1 and BRCA2 .BRCA1 was cloned in 1994 and codes for a protein involved in transacription. A woman with one of these genes has about an 80% risk of developing breast cancer before the age of 70.

Ionizing radiation: This included X-rays, ỵ-ray and particle from the decay of radioactive elements. Cancers were caused in workers with X-rays at the beginning of the twentieth century and factory workers painting the dials of watches with a luminous paint containing radioactive radium and thorium. The radiation causes the formation of chemical active and damaging ions inside cells which can break DNA strands or cause mutations. The types of cancer linked with ionizing radiation include skin cancer, bone marrow cancer, lung cancer and breast cancer. Medical and dental X-rays also expose patients to ionizing radiation.

Ultraviolet light: This is the most common form of carcinogenic radiation and is non-ionising. DNA absorb ultraviolet light and the energy is used in converting the bases into more reactive forms which react with surrounding molecules. Sunlight contains ultraviolet light and prolonged exposure to it can result in skin light and prolonged exposure to it can result in skin cancers, including melanoma which is highly malignant and commonly cause death through secondary brain tumours. Depletion of the ozone layer result in a higher proportions of ultraviolet light reaching the Earth’s surface. The brown skin pigment melanin offers some protection.

Radon gas: Radon gas is a natural source of radiation released from certain rocks such as granite. It may accumulate in houses in areas where these rocks are found. It has been linked to the development of leukaemia (cancer of white blood cell), lung, kidney and prostate cancers, although the evidence is inconclusive.

Chemical mutagens: Many chemical are now recognized as causing cancer. The first example was described in 1775 as soot and coal tar, when chimney sweeps were discovered to develop cancer of the scrotum. Later mineral oils were also found to be carcinogenic, when mills. The workers developed cancers of the abdominal synthetic dye industry in the late nineteenth century developed bladder cancer.
This list of chemical carcinogens has steadily lengthened over the last 90 years and now includes, in addition to the above, inorganic arsenic compounds which produce skin cancer and asbestos products which cause lung cancer. Some food additives (flavours, colourings and stablisers) have been considered as possible carcinogens because they cause cancers in experimental animals. As a result a number have been withdrawn.

Tobacco smoke contains chemical responsible for ling cancer. The most important of these are carcinogens. Many common foods contains carcinogenic chemical although the levels are mostly low.