Definition of Viruses

Viruses are complexes consisting of protein and an RNA or DNA genome. They lack both cellular structure and independent metabolic processes. They replicate solely by exploiting living cells based on the information in the viral genome.

Viruses are autonomous infectious particles that differ widely from other microorganisms in a number of characteristics: they have no cellular struc­ture, consisting only of proteins and nucleic acid (DNA or RNA). They have no metabolic systems of their own, but rather depend on the synthetic mecha­nism of a living host cell, whereby the viruses exploit normal cellular meta­bolism by delivering their own genetic information, i.e., nucleic acid, into the host cell. The host cell accepts the nucleic acid and proceeds to produce the components of new viruses in accordance with the genetic information it contains. One thus might call viruses “vagabond genes."

Viruses infect bacteria (so-called bacteriophages), plants, animals, and humans. The following pages cover mainly the human pathogen viruses (Fig. 1). Table 1 lists the essential characteristics of viruses.

Table 1 Essential Characteristics of Viruses

Fig. 1 Different virus species are shown here to scale inside an E. coli bacterium.

References

Fritz H. Kayser, M.D. Emeritus Professor of Medical Microbiology Institute of Medical Microbiology, University of Zurich, Zurich, SwitzerlandThieme 2005, Stuttgart ! New York.