What are Viruses?

• •  Infectious acellular agents i.e. devoid of cell components like nucleus, organelles, cytoplasm, or plasma membrane

•  Replicate or multiply only inside living host cell

•  Hence, also called obligate intracellular parasites

•  Possesses only one type of nucleic acids- either DNA or RNA but never both (exception is cytomegalovirus)

Fig. 1. Virions of mimivirus, one of the largest viruses and a parvovirus (arrowed), one of the smallest viruses.

History of Virology