Introduction to virus structure, classification, replication, and hosts

Viruses are the smallest currently known organisms. All are obligate parasites; thus, they have absolute requirement to grow in host cells, and, in many cases, can only grow in particular tissue types and/or in specific types of hosts. For example, viruses that infect vertebrate animals cannot infect plants and vice versa, although a few viruses can grow in insects and in either plants or animals. Because no virus can replicate by itself, the need to use a host cell often results in disease manifestation. Several characteristics differentiate viruses from all other known organisms. All known cellular life forms have their genetic information as DNA, and this DNA is copied into RNA for protein translation. While some viruses also have DNA genomes, many viruses have an RNA genome and never make or use DNA. Of more fundamental difference, all cellular life forms replicate by binary division, whereas viruses are spontaneously assembled from their component parts.