We don't always know. Certain things can lead to the cell mutations that result in cancer. These cancer-causing agents are called carcinogens. Some like tobacco and asbestos are known. However, there are others we are not yet sure about, mainly because it is often years after contact with the carcinogen that a cancer starts or the symptoms appear.
You can reduce your risk of developing many of the most common cancers. For example, tobacco smoking causes lung cancer and tobacco chewing causes mouth cancer, so these cancers are much less likely if you don't use tobacco. A simple test to identify cell changes at an early stage can reduce the likelihood of women developing cervical cancer. Reducing exposure to ultraviolet radiation can lower your risk of developing skin cancer.
Watching your diet, maintaining a healthy weight and being physically active can also help. Eating a diet high in plant foods — vegetables, fruit and wholegrain cereals — and cutting down on refined products, sugar, salt and animal fat will improve your health and reduce your risk of developing some cancers, such as bowel cancer.
Cancer treatment depends on the type of cancer. Some are best treated by surgery to remove the tumour, others with radiotherapy, which uses various kinds of radiation to kill the cancer cells or to stop them dividing. Other cancers are best treated by chemotherapy, where special drugs are used to kill the cancer cells. Often a mixture of two or more of these methods is used.
