Cancer happens when normal cells change so that they grow in an uncontrolled way. The uncontrolled growth causes a tumor to form. If not treated, the tumor can cause problems by taking over the normal tissues nearby or by causing pressure on other body structures.
Cancer has over 200 different diseases, not just one. The difficulty is that different cancers are also caused by different things, so no one can really prevent them, and different cancers respond to different treatments so no one treatment can cure them all. Some cancers are curable though like, Testicular cancer, Hodgkin’s disease, and many cases of leukemia can all be cured in adults with chemotherapy, most skin cancers are cured with surgery, and many cases of thyroid cancer are cured with radiotherapy.
There is a lot of research going on in all the types of cancer to try and find a cure, scientists are also researching on how cancers grow their own blood supplies and researchers are beginning to test a drug that may be able to stop that happening and so stop cancers from growing.