Franklin Watts, 1976. Harper Collins Dictionary of Biology . Harper Collins, 1991. Viruses are some of the smallest non-cellular organisms known to man. These simple parasites are composed of nucleic acid and a protein coat.
Most viruses are smaller than eukaryotic cells and even bacteria.
Methods of studying viruses have changed over time. One method is to place virus particles in a solution of bacteria then wait until the virus multiplies and takes over. There could be various reasons for the contrast in information, including the sheer number of viruses. There are hundreds of viruses that are responsible for the common cold alone. With so many, recorded sizes may vary according to the variety.