Infectious agents are second only to tobacco use as a potentially preventable cause of cancer in humans. Estimates vary between 15% and 30% as to the percentage of cancers worldwide that are associated with an infectious etiology. ^1,2 The burden is greater in the developing world, but the impact even in the United States and other developed countries is significant. Specific viruses, parasites, and bacteria are now associated with specific human cancers. These are discussed in some detail in this chapter.