capsid
22.1 Introduction Viruses are the smallest biological particle the tiniest are only 20 nm in diameter . However, they are not biological organisms so they are not classified in any kingdom of living things. They do not have any organelles and cannot respire or perform metabolic functions. Viruses are merely strands of DNA or RNA surrounded by a protective protein coat called a capsid. Viruses only come to life when they have invaded a cell. Outside of a host cell, viruses are completely inert. Since first being identified in 1935, viruses have been classified into more than 160 major groups. Viruses are classified based on their shape, replication properties, and the diseases that they cause. Furthermore, the shape of a virus is determined by the type and arrangement of proteins in its capsid. Viruses pathogenic to humans are currently classified into 21 groups. Viruses can also attack bacteria and infect bacterial cells. Such viruses are called bacteriophages.