Some kinds of cancer are caused by things people do. Smoking can cause cancers of the lungs, mouth, throat, bladder, kidneys and several other organs, as well as heart disease and stroke. While not everyone who smokes will get cancer, smoking increases a person's chance of getting the disease. Drinking a lot of alcohol has also been shown to increase a person's chance of getting cancer of the mouth, throat, and some other organs. This is especially true if the person drinks and smokes.
Radiation (x-rays) can cause cancer. But the x-rays used by the doctor or dentist are safe. Too much exposure to sunlight without any protection can cause skin cancer.
In many cases, the exact cause of cancer remains a mystery. We know that certain changes in our cells can cause cancer to start, but we don't yet know exactly how this happens. Many scientists are studying this problem.