Cancer is not considered a single disease. It is a complex and large group of malignancies which can affect every organ in your body. Cancer cells reproduce themselves at a rapid rate unlike normal cell production. If the cells metastasize, they can attack different organs. These cells then break off and travel through the blood stream to other parts of the body and begin metastasize.
Cancer can be caused by both external and internal factors. Exposure to carcinogens poisons, pesticides, etc. can cause cancer as well as hormones and inherited DNA mutations can create cancerous cells in the body.