What is cancer and what causes cancer?
Cancer is a generic term for a large group of diseases that can affect any part of the body. Other terms used are malignant tumors and neoplasms. One defining feature of cancer is the rapid creation of abnormal cells that grow beyond their usual boundaries, and which can then invade adjoining parts of the body and spread to other organs, the latter process is referred to as metastasizing. Metastases are a major cause of death from cancer.

Cancer arises from the transformation of normal cells into tumor cells in a multistage process that generally progresses from a pre-cancerous lesion to a malignant tumor. These changes are the result of the interaction between a person's genetic factors and 3 categories of external agents, including:

• physical carcinogens, such as ultraviolet and ionizing radiation;
• chemical carcinogens, such as asbestos, components of tobacco smoke, aflatoxin (a food contaminant), and arsenic (a drinking water contaminant); and
• biological carcinogens, such as infections from certain viruses, bacteria, or parasites.

Ageing is another fundamental factor for the development of cancer. The incidence of cancer rises dramatically with age, most likely due to a build-up of risks for specific cancers that increase with age. The overall risk accumulation is combined with the tendency for cellular repair mechanisms to be less effective as a person grows older.

Risk factors for cancers; modify and avoid them
Tobacco use, alcohol use, unhealthy diet, and physical inactivity are major cancer risk factors worldwide. 

Modifying or avoiding key risk factors can significantly reduce the burden of cancer. These risk factors include:

• Tobacco use including cigarettes and smokeless tobacco.
• Being overweight or obese.
• Unhealthy diet with low fruit and vegetable intake.
• Lack of physical activity.
• Alcohol use.
• Sexually transmitted HPV-infection.
• Infection by hepatitis or other carcinogenic infections.
• Ionizing and ultraviolet radiation.
• Urban air pollution.
• Indoor smoke from household use of solid fuels.

Tobacco use is the single most important risk factor for cancer and is responsible for approximately 22% of cancer-related deaths globally (2).