Principles of Epidemiology and Microbiology
Lesson 2: Public Health Microbiology
Section III: Viruses
2-13
2-13. MORPHOLOGY AND PHYSIOLOGY OF VIRUSES
a. Morphology. Because of its minute size, a virus must be studied with the electron microscope. The virus is much simpler than the bacterial cell, consisting of a core of nucleic acid--either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA)--enclosed in a protective membrane of protein (called the capsid). Some viruses have a lipid membrane called an envelope around the outside of the capsid. The shape of viruses varies widely, but in general, they tend to conform to the following:
(1) Viruses of humans, animals, and plants are spherical rod-shaped, or many sided. (Figure 2-7).
(2) Viruses of bacteria (bacteriophages) are shaped like a lunarlander spacecraft. (Figure 2-7).
b. Physiology. Being an obligate parasite, the virus requires a living host cell for existence. When a virus comes into contact with a susceptible host cell, it becomes attached to the surface. The virus or its nucleic acid then enters the host cell, where it takes control and utilizes the protoplasm of the host cell to produce new virus particles. The new virus particles then invade additional host cells, and multiplication continues. There is no cell division in the growth and reproduction of viruses. A new virus is formed by chemical synthesis of the viral nucleic acid and capsid proteins (replication). Host cells that have been attacked by a virus may be completely destroyed, or they may suffer little or no harm.
Figure 2-7. Virus forms.
c. Pathogenesis. Many virus infections are asymptomatic. On the other hand, some of the most dangerous and highly contagious diseases are caused by viruses. One characteristic of some viruses is found in no ‘other known living disease agent. That characteristic is the ability to cause host cells to proliferate in an abnormal manner, forming growths or tumors (hyperplasia). It is this characteristic which underlies the current speculation that some cancers may be caused by a virus. Some viruses cause the development of inclusion bodies in the cells they attack. These inclusion bodies may be colonies of the virus, or they may be products formed by the cell in response to the attack. The presence of inclusion bodies is very important in the diagnosis of certain diseases where it is not possible to isolate the virus. In rabies, for example, the presence of Negri bodies (named for their discoverer) in brain cells has been used to confirm the diagnosis, even though the rabies virus may not be isolated.
