Everyone knows about cancer, and many people have a hard time saying the exact definition of cancer, but the word often sends us into mourning. What exactly is cancer? According to the World Health Organization, cancer, also known as malignant neoplasm, is a group of diseases caused by the uncontrolled growth of cells that proliferate and invade the surrounding normal tissues and even metastasize to other parts of the body through the body’s circulatory system. It is understood as a primitive, self-centered (without regard to the big picture), destructive, unconstructive cell that divides and multiplies out of control, swamping normal, healthy and useful cells. Not all tumors are cancerous, and benign tumors do not spread to other parts of the body. So what exactly is cancer? This is a question that is both ridiculous and helpless. If we cannot answer this question, we have no way to understand cancer, much less to prevent and treat it effectively. Just like our other diseases, bronchitis occurs in the tracheal tissues and heart disease occurs in the heart, cancer is also classified in the same way. Those that occur in epithelial tissues are generally called carcinomas and those that occur in mesenchymal tissues are called sarcomas. For example, breast cancer is a malignant tumor on the breast, and cancer cells found in the liver are called hepatocellular carcinoma. Also some tumors may have special names, such as lymphoma and leukemia. Cancer can occur in any part of our body, from head to toe, and cancer cells can also spread to any part of the body. A tumor can be thought of as a genomic disease. When a tumor occurs, first of all our genome, so to speak, a large number of genes are mutated, and this change may require some external factors, such as chemical pollution, biological factors, radiation, etc. Then these mutated cells partially escape our body’s immune system to survive and become “immortal” cells and keep proliferating. Some of these cells do not rest on their laurels and roam around, following the blood and lymphatic system to all parts of the body. Here some people may ask, is there any hereditary tumor? Will they be contagious? First of all, let’s say that the vast majority of tumors do not have very typical genetic characteristics, but about 5-10% of cancer patients do carry some known genetic factors, such as the BRCA1 and BRCA2 mutations that cause breast cancer. Families with this gene mutation tend to have a high incidence of breast cancer and also have ovarian cancer. Hollywood star Angelina Jolie has undergone preventive mastectomy and breast cancer treatment. Jolie underwent a prophylactic mastectomy and an oophorectomy because her mother gave her the mutated BRCA1, which resulted in an 87% and 50% chance of developing breast and ovarian cancer. Second, as of today, cancer has not been found to be transmissible between humans. According to the World Health Organization survey, 1/3 of cancers can be prevented, 1/3 can be detected early and cured with timely treatment, and only 1/3 of cancers cannot be cured. Knowing cancer, understanding cancer, cancer is not scary, cancer is preventable and curable.