Introduction to Animal Viruses

Viruses are infectious agents that are not cellular in nature. They consist of a nucleic acid genome packaged within a protein shell. Although relatively simple, viruses exhibit significant diversity in terms of size, genome organization, and capsid architecture. All viruses are obligate intracellular parasites as they must obtain energy and building blocks from the cell. They subvert many host cell processes for their own replication, and studying virus replication has provided detailed information about the basic workings of cells. At the cellular level, possible outcomes of infection range from production of virus particles without damage to the cell, to cell death, or occasionally cell transformation. In humans and animals the outcomes of infection range from inapparent (no disease) to considerable disease and death. Some viruses cause acute infections lasting for days or a few weeks while others infect their hosts for a lifetime. Viruses can evolve and rapidly adapt to changing conditions. Some viruses are easily replicated in cultured cells, while others require specific conditions only found in specialized cells within a human or animal. While this text focuses on viruses of humans and other animals, viruses infect organisms of all types, from bacteria to fungi to plants. Viruses are most often classified based on groups of genome and virion characteristics. Genome sequence comparisons provide an unbiased method for grouping and categorizing viruses.