Hosts and size Three main classes -animal viruses. bacterial viruses (bacteriophages), and plant viruses The particular host range of a virus is determined by the virus's requirements for its specific attachment to the host cell and the availability within the potential host of cellular factors required for viral multiplication Virus particles(virions) vary widely in size and shape. Viruses are smaller than cells, ranging in size from 0.02 to 0.3 um. A common unit of measure for viruses is the nanometer which is 1000 times smaller than 1 um and 1 million times smaller than 1 mm. Smallpox virus, one of the largest viruses, is about 200 nm in diameter(a bit smaller than the size of the smallest bacteria); poliovirus, one of the smallest, is only 28 nm in diameter (about the size of a ribosome)Hosts and size Three main classes - animal viruses, bacterial viruses (bacteriophages), and plant viruses. The particular host range of a virus is determined by the virus's requirements for its specific attachment to the host cell and the availability within the potential host of cellular factors required for viral multiplication. Virus particles (virions) vary widely in size and shape. Viruses are smaller than cells, ranging in size from 0.02 to 0.3 μm. A common unit of measure for viruses is the nanometer, which is 1000 times smaller than 1 um and 1 million times smaller than 1 mm. Smallpox virus, one of the largest viruses, is about 200 nm in diameter (a bit smaller than the size of the smallest bacteria); poliovirus, one of the smallest, is only 28 nm in diameter (about the size of a ribosome)