Scientists aren't sure of the mechanism through which excessive weight increases the risk of the disease or if it can be reduced if people lose weight. There is now a consensus that cancer is commoner in those who are overweight. Infections and a weakened immune system are other known risk factors for cancer, along with excessive alcohol, sunlight and pollution. Helicobacter pylori , a bacteria linked to ulcers and stomach cancer, and the human papilloma virus found in most cervical cancers are among infections that contribute to about 15 percent of cancers. Doctors also suspect childhood leukemia is triggered by an infection, but scientists have not pinpointed the cause.
Viruses are particularly important in the developing world.
Genetic susceptibility is also a risk factor in many types of cancer but Peto added that most of the genes involved remain to be discovered. All rights reserved. This material may not be published, broadcast, rewritten or redistributed. ABC News Internet Ventures. Internet Safety Information applicable to the site.