The genes control the cell. They decide when it will reproduce, what it does and even when it will die. Normally the genes make sure that cells grow and reproduce in an orderly and controlled way. If the system goes wrong for any reason, the usual result is that the cell dies. Rarely, the system goes wrong in a way that allows a cell to keep on dividing until a lump called a 'tumour' is formed.
Cancers can cause different symptoms in different people because of where they are.
A cancer may press on a nerve, or another body organ that is nearby. The place where the cancer starts also affects what treatment can be used because doctors have to take into account the risk of damaging neighbouring organs. There are about 200 different types of cancer affecting the different body tissues. What affects one body tissue may not affect another. For example, over exposing your skin to the sun could give you a melanoma on your leg.