virions come in many sizes and shapes. most viruses are smaller than prokaryotic cells, ranging in size from 0.02 to 0.3 um (20-300 nm). A common unit of measure for viruses is the nanometer, which is one-thousandth of a micrometer. Smallpox virus, one of the largest viruses, is about 200 nm in diameter about the size of the smallest cells of bacteria . Poliovirus, one of the smallest viruses, is only 28 nm in diameter (about the size of a ribosome. Consequently, viruses could not be properly characterized until the invention of the electron microscope in the 1930s