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Viruses and the increased risk of human cancer.

Sinisa Franjic

Viruses are the smallest parasites, ranging in size from 0.02 to 0.3 μm. In order to reproduce, they are completely dependent on the cell. Viruses have an outer protein and sometimes lipid envelope and RNA or DNA. In order for an infection to occur, the virus is first attached to the host cell. The viral DNA or RNA is then separated from the outer envelope and replicated within the host cell, an event that requires specific enzymes. Most RNA viruses replicate their nucleic acid in the cytoplasm, while DNA viruses do so within the nucleus. The host cell typically decays, releasing new viruses, which infect other host cells. Several hundred different viruses infect humans. Viruses that primarily infect humans are often spread by respiratory and digestive secretions. Some are transmitted sexually and through blood. Viruses are widespread throughout the world, but their spread is limited by innate resistance, previous immunizing infections or vaccinations, sanitary and other public health control measures, and the prophylactic use of antiviral drugs.