There is an excessive production of abnormal cells. These cells go into the surrounding tissues and cause damage. Through blood lymph vessels these cells may go to distant parts of the body and make new colonies of proliferating cells. When germs or bacteria invade body tissues they cause an infection. The body launches defense mechanisms against these foreign particles and destroys them.
Cancer is due to proliferation of abnormal cells, which develop from body's own cells.
So they are not recognized as foreign and continue to grow and invade without being attacked by body defenses. Cancers become dangerous if they are detected late, that is at a stage when they have spread to other organs or have caused destruction to surrounding tissues. The secondary growths interferes with the functioning of the particular organ or site involved. On the other hand, If the cancer is detected at an early stage, it can often be cured either by means of surgery or be other therapeutic means available. No single cause has been responsible for all cancers.