Fundamentals of Microbiology

1.5.1 General characteristics of viruses

Viruses may be regarded as exceptionally complex aggregations of nonliving chemicals or exceptionally simple living microbes. Because viruses are inert outside living host cells but once viruses enter a host cell, they become active and viral multiplication occurs. Therefore viruses can be termed as obligatory intracellular parasites. Virus is the Latin word for poison.

Viruses contain a single type of nucleic acid (either DNA or RNA) and a protein coat, sometimes enclosed by an envelope composed of lipids, proteins and carbohydrates. Viruses are obligatory intracellular parasites. They multiply by the host cells synthesizing mechanisms. A complete, fully developed viral particle composed of nucleic acid surrounded by a coat is called a Virion.

Host range:

Host range refers to the spectrum of host cells in which a virus can multiply. Viruses are host specific. There are viruses that infect invertebrates, vertebrates, plants, protists, fungi and bacteria. Most viruses infect specific types of cells of only one host species. Host range is determined by the specific attachment site on the host cells’ surface and the availability of host cellular factors.

Size:

Viral size is measured by electron microscopy. Viruses range from 20 nm to 300 nm in length.