Cancer is rarely triggered by a single factor. It usually results from an interplay between environmental factors - such as carcinogens or viruses - on one hand, and factors within the body - such as hormones and inherited genes - on the other. Among numerous carcinogens, tobacco towers over the rest as the biggest killer. In the US, for example, it is responsible for 30% of all cancer deaths and 87% of lung cancer deaths. Researchers estimate that a further one-third of cancer deaths can be blamed on other "lifestyle" factors, such as diet and obesity.
Viruses can cause cancer by interfering with the genetic instructions in cells. The worst culprits for this are the hepatitis B virus, which can cause liver cancer, and the human papilloma virus, which can cause cervical cancer. The risk of developing cancer increases with age, simply because cancerous changes in cells usually take years to occur. Two of every three people diagnosed with cancer in the UK each year are over the age of 65.