Cecil's Textbook of Medicine. . The common cold or Acute Coryza. 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.