Viruses are genetic entities that lie somewhere in the gray area between living and non-living states. Viruses consist of genetic material (DNA or RNA) encased in a protective protein shell, come in many different shapes and are many times smaller than bacteria ? ranging 20 to 400 nanometers in diameter. As viruses lack the biosynthetic machinery that is necessary for metabolic function and reproduction, in order for a virus to replicate it must find and infect a suitable host cell, which then mistakes the virus for a protein and replicates its genetic code. These viral infections often cause diseases in humans and animals. Learn more about viruses, their types, life-cycles and effects on human health in this book.