Cancer is a group of diseases characterized by uncontrolled growth and spread of abnormal cells. Cancer is caused by external factors and internal factors which may act together to initiate or promote carcinogenesis. External Factors - chemicals, radiation, viruses, and lifestyle. Internal Factors – hormones, immune condition, and inherited mutations. Oncology branch of medicine deals with etiology, diagnosis, treatment and prevention of cancer. Onco - is a Greek word meaning tumor .