There are at present 20 recognized families of viruses that contain viruses of humans or animals. Historically, properties used most often in classification derive from electron microscopic examination e.g. virion morphology, structure and dimensions, and presence or absence of an envelope.
Additional characterization is supplied by the type and nature of the genomic nucleic acid.
A few of the more common animal viruses will be illustrated. DNA-containing viruses 1. Hepatitis B virus 5. Molluscum contagiosum 6. Parvo virus RNA-containing viruses 1.