We have found a load of different chemicals that cause cancer, for example the ones in cigarette smoke. They cause cancer by attacking our cells in our bodies and damaging our DNA. It’s pretty hard to stop that from happening on a molecular level, but we can stop the exposure to these chemicals in the first place, say for example, quitting smoking. Once a person has cancer, the medicines we have work by attacking the cancer cells specifically, which is really hard to do without attacking everything else in your body. We’ve already come a long way from where we were, and many people do get cured from cancer. There’s still a long way to go though.