Cells grow then divide by mitosis only when we need new ones - when we're growing or need to replace old or damaged cells.
When a cell becomes cancerous, it begins to grow and divide uncontrollably. New cells are produced - even if the body does not need them.
A group of cancerous cells produces a growth called a tumour.
Cancer cells are undifferentiated - they do not carry out their normal function. The diagram shows how cancer cells can invade surrounding tissue:
There are genetic factors that increase the likelihood of developing some cancers.
Chemicals and other agents that can cause cancer are called carcinogens.
Carcinogens cause cancer by damaging DNA. Carcinogens cause mutations to occur. A single mutation will not cause cancer - several are required. For this reason, we are more likely to develop cancer as we get older.
Something that increases the likelihood of developing a disease is called a risk factor. There are several risk factors for various types of cancers including:
There are also genetic risk factors for some cancers.