[문 18～문 20] Cancer can occur in any kind of cell. Since there are many different kinds of cells, there are many kinds of cancer. So cancer is not one disease but a large family of diseases. This is one of the problems in finding a cure for cancer. One approach in dealing with the problem of cancer is. Scientists have found many cancer-causing agents that are chemicals. Steps have been taken by governments to keep such chemicals out of food and to prevent other forms of contact with them. On the other hand, because of the close links between cancer and viruses in certain animals, more and more scientists are coming to believe that many types of cancer are caused by viruses. But exactly how a virus can produce cancer in the human body is still not known.