Cancer is a dangerous disease in which certain cells in our body grow in an uncontrolled way. It is one of the world’s most serious illnesses. Together with heart attacks, it kills more people than any other disease in the world.

The human body has billions of cells. They are tiny elements of living material. Cells always reproduce themselves. Normally our body controls this process. It tells cells to divide themselves when we need it and to stop when we don't. Sometimes, however, cell growth gets out of control and the production of cells doesn't stop.

These cells that produce new tissue are called tumours. They can be benign (good) tumours or malignant(bad) tumours. A benign tumour usually stays in the same area in which it starts growing. It is often harmless. A malignant tumour however is dangerous. It can grow and spread to healthy cells and destroy them. Cells from malignant tumours can also spread to other parts of the body and produce more tumours. These malignant tumours are the ones that cause cancer and may even lead to death. Sometimes they enter the blood and lymphatic system. When this happens, cancer metasizes.

Our body's lymphatic system

Image: (modified version) File: Illu lymphatic system.jpg, CC BY 3.0, via Wikimedia Commons