Cancer is caused by the uncontrolled division of cells. If a cancer is benign, it will be a certain size and stay that size. However if it is malignant, then it will grow and get bigger in size. Eventually it will reach the blood stream. The cancer cells can travel along the blood to other parts of the body and become established there. So if the stomach cancer caused by Helicobacter pylori is malignant and has grown so much that it has reached the blood, its cells can spread to other parts of the body by travelling along the blood stream. Once they reach other parts of the body, such as the liver, they form secondary tumours in those places.