Viruses

Submicroscopic infectious agents

similarities and differences

Viruses differ fundamentally from true microorganisms and organisms in their composition and reproduction. Due to a lack of their own metabolism, viruses are obligate and depend on reproduction by host organisms. Viral reproduction does not take place through growth and division, but through replication of the viral nucleic acid and synthesis of the virus-specific proteins and their subsequent assembly into mature, infectious virus particles.  Viruses can contain either DNA (deoxyribonucleic acids; DNA viruses) or RNA (ribonucleic acids; RNA viruses), which controls virus replication inside the host cell (virus infection) and is always surrounded by a protein envelope (capsid) outside the host cell, in the released virus particle (virion). As a virion, the viral nucleic acid is transported extracellularly from one cell to another and from one host organism to another.  Depending on their host specificity, viruses are divided into bacterial viruses (bacteriophages), algal viruses, fungal viruses, plant viruses and animal viruses (containing vertebrate viruses [vertebrate viruses], human pathogenic viruses [human viruses], invertebrate viruses and insect viruses [arthropod viruses]).