Entire virus particle, consisting of an outer protein shell (called a capsid) and an inner core of nucleic acid (either RNA or DNA ).
The core gives the virus infectivity, and the capsid provides specificity (i.e., determines which organisms the virus can infect). In virions whose capsids are further encased by a fatty membrane, the virion can be inactivated by exposure to a solvent such as ether or chloroform. Many virions have capsids with 20 triangular faces and the nucleic acid densely coiled within; others have capsids consisting of surface spikes, with the nucleic acid loosely coiled within. Virions of most plant viruses are rod-shaped.