How does cancer make you ill?

Cancerous cells divide uncontrollably. Usually they form a lump of tissue called a tumour. The exception to this is leukaemia, where the cancerous white cells do not form a lump but instead multiply in the bloodstream, stopping the blood from functioning normally.

On its own a single tumour can make you quite ill, for example by pressing against nerves, blocking the digestive system or blood vessels, or by releasing hormones that can affect the normal workings of your body.

Things can get much worse, however, if the cancer becomes malignant and spreads. This happens when a single cancerous cell breaks away from the main tumour and starts growing and dividing elsewhere in the body.

To do this, it must acquire more mutations that allow it to survive in other parts of the body. For a cancer cell to spread, it must be able to do two things that the normal cell it grew from could not:

• it must be able to leave its usual environment and travel through the blood or lymph system, a process called invasion.
• when it arrives at its new location, it must be able to make new blood vessels grow around it and supply it with oxygen and nutrients, a process known as angiogenesis.

The whole process of a tumour gaining new mutations as it divides and then spreading to other parts of the body is called metastasis. Once a tumour metastasises, it is much more difficult to treat, and can make you much more ill because more areas of the body are affected. Different types of cancer often 'prefer' to spread to other sites in the body. For example, prostate cancer often spreads to the bones.

Sometimes cells can divide uncontrollably without being able to spread. These cells form a benign tumour, which can be easily removed with surgery.