What is Cancer?

The Canadian Cancer Society states:

All types of cancer start in our cells. Our bodies are made up of trillions of cells grouped together to form tissues and organs such as muscles, bones, the lungs and the liver. Genes inside each cell tell it when to grow, work, divide and die. Normally, our cells follow these instructions and we stay healthy. But sometimes the instructions get mixed up, causing our cells to grow and divide out of control or not die when they should. As more and more of these abnormal cells grow and divide, they can form a lump in the body called a tumour.

Cancer 101