Cancer, is a malignant tumor. Cancer cells are not foreign to the body. The first cancer cells, which are formed by the change of normal cells in the body. One cancer cell, multiplying into billions of cancer cells, eventually becomes a visible tumor. Tumor has infinite growth habit and needs to absorb nutrients, so it takes away nutrients from neighboring cells and tissues, thus affecting the growth of neighboring cells and tissues, and the tumor is bound to invade neighboring tissues and affect the function of neighboring tissues. The normal function and order of human tissues are thus disturbed. Why do normal cells become cancer cells? It is genes, more specifically, mutations in genes. Cancer is a genetic disease Every cell has a nucleus, which contains genetic material. Initially, scientists stained the cells in order to see them under the microscope, and the dyed cells had some rod-shaped substances that were darker in color, which scientists called chromosomes. Later research found that chromosomes are genetic material, each chromosome is a very long curly molecule called deoxyribonucleic acid, the English abbreviation for DNA, a fragment of DNA is a gene, a DNA has hundreds and thousands of genes, all the DNA in a cell contains tens of thousands of genes. All human traits are determined by genes, either by a single gene or by multiple genes. Every day, about 1 trillion cells die in the human body, while an equal number of cells are reborn, and the new cells replace the dead ones. How are cells reborn? It is created by splitting existing cells in two. Where do the dead cells go? Most of the dead cells are engulfed by macrophages, which are like nature’s carrion eaters, responsible for cleaning up the carcasses of other animals. Some of the dead cells are excreted out of the body with metabolism. Some dead cells decomposed themselves and were absorbed by other cells as nutrition. In the massive “replacement” of dead cells by new cells, one of the top cells may be a cancerous cell. How does a cell become cancerous? It is the mutation of a gene that causes the cell to become cancerous. There are two types of gene mutations: in the first case, some people are born with a certain gene; in the other case, the gene is changed under the stimulation of certain carcinogenic substances. The changes of genes in both cases may prompt the cells to become cancerous. Therefore, cancer is a genetic disease. However, it can never be said that cancer is a genetic disease because the genes that prompt the cells to become cancerous are changed later in life, and such changes are not inherited from the previous generation and are not passed on to the next generation. In fact, cancer is also a disease of life, because life is full of carcinogenic substances and carcinogenic factors. Human defense system The human body is not helpless in the face of cancer cells. The human body has its own defense system, which is the immune system. The function of the immune system is to identify the enemy, attack the enemy and destroy the enemy. The human body’s immune system can attack not only bacteria, viruses and other foreign substances, but also cancer cells. During the whole decades of a person’s life, many cells in the body may become cancerous, but not every cancerous cell will develop into cancer. Some cancer cells are destroyed by the immune system and some die naturally. Yes, cancer cells can die naturally just like normal cells. Why do cancer cells spread and metastasize? Cancer cells are very restless in the human body and will spread and metastasize to other organs, which is related to the growth mode and characteristics of cancer cells. The reasons can be summarized as follows: Firstly, cancer cells multiply rapidly, and because of the rapid increase in number, the original space cannot accommodate so many cells, so the cells at the edge of the tumor are squeezed into the surrounding tissues. Therefore, some cancer patients can have one operation and still cannot cut the tumor completely clean. Secondly, the special chemical composition and structure of the surface of cancer cells make the adhesion between cancer cells low and the connection loose, which can easily detach from the cancer mass and create conditions for the spread. Thirdly, cancer cells secrete special substances that dissolve and destroy surrounding tissues, opening the way for spreading and metastasis. Fourthly, cancer cells contain a special substance that can promote the formation of blood clots. After cancer cells enter blood vessels, they promote the formation of blood clots, obstruct blood flow and resist blood flow flushing, so that they can attach themselves to the walls of blood vessels or other parts and continue to grow. In addition to the above reasons, there may be some other factors that have not yet been recognized and understood.