Viruses are not capable of independent metabolism. Viruses must invade a host cell in order to gain the components needed for almost all biological functions required for their survival.
Viruses are among the smallest biological entities.
They range in size from 20 to 40nm, comparatively much smaller than the typical bacterium. Before the advent of electron microscopes viruses were unable to be seen. The capsid is made up of repeating protein units called protomers. Together the nucleic acid core and the capsid are called a nucleocapsid. In some instances, the nucleocapsid affects infectivity.