Infectious Agents Associated With Cancer

In 2002, it was estimated that almost 18% of cancers worldwide (approximately 1.9 million cases) were associated with an infectious etiology, whether it be a virus, bacteria, or parasite. The idea of an infectious agent as a carcinogen is based on the understanding that the infection contributes to the development of malignancy through carcinogenesis by a combination of genetic alteration of the cell and/or impairment of immune function. Typically, cancer-associated infectious agents are characterized by the establishment of chronic rather than acute infections, and a prolonged period of time between the onset of infection and development of malignancy. However, the majority of those infected do not ultimately develop malignancy.

A period of prolonged infection can establish an environment for carcinogenesis through three broad mechanisms: virus-induced transformation (by either causing alterations in the regulation of the cell cycle including activation of oncogenes or inhibition of tumor suppressors), local effects of chronic inflammation (leading to higher proliferation of tissue, direct DNA and cell membrane damage, and increased potential for DNA mutagenesis through release of cytokines and reactive oxygen species), or immunosuppression (altered cytotoxic lymphocyte responses and inability to manage malignant precursor cells). Infectious agents associated with human cancers are listed in Table 9.2.

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