A tumour results from the inexorable multiplication of a single cell in the body, triggered by inadvertant damage to critical genes within that cell. By chance, some of the damage will occur in the critical genes that normally tightly control the proliferation and survival of cells.
Cancer is predominantly a disease of aging but paradoxically also occurs in young children.
Most cancers of the young originate in inherited mutations within this same group of genes. The difference often is that the child has inherited a single mutation from a parent, greatly increasing the risk that further accidental mutations will trigger cancer early in life. However, only a small minority of cancers are thought to arise from inherited mutations. Instead, most tumours develop due to random mutations within the various cells of the body - a cruel lottery. Others provoke inappropriate gene activity.