(Tobacco Mosaic Virus) followed by foot and mouth disease virus in 1898, the first

human virus was discovered only in 1901 as the yellow fever virus. But scientists did

not see an actual virus until 1930. In 1915, Frederick Twort, a bacteriologist, dis-

covered a bacteriophage, the virus that can infect bacteria, notifying it as a micro-

organism that would kill bacteria. Hence, it established a unique feature of the virus as

their size could vary within a range between 20 nm to 1 micron. They are much

smaller than the cells they could infect.

2.1.2

VIRUS CHARACTERISTICS

Viruses are among the most symmetrical biological objects. They can be either

helical, spherical, icosahedral, or have more complex structure. They could be fi-

lamentous with elongated structures. Viruses can be visualized by x-ray crystal-

lography or electron microscopy. Looking at the structure of TMV, it has given the

concept that viruses are structurally composed of repeating subunits. The viral

structure consists of some key features; namely, the capsid encapsulating the viral

genome. The virus may or may not have an envelope layer.

2.1.3

VIRUS EVOLUTION AND CLASSIFICATION

Evolution of viruses has remained very speculative as they do not fossilize. They do

not have a common ancestor. There have been different theories of virus evolution.

First is the devolution or regressive theory, meaning they could have originated

from free-living cells. Second is the escapist or progressive theory, which explained

they might have originated from RNA and DNA molecules that escaped from the

host cell. The third is the self-replicating theory, explaining a system of self-

replication involving evolution alongside the host cell.

Viruses are classified based on their morphology, chemical composition, host

organism, or mode of replication. But, since the discovery of viruses, the classification

system has been modified to the system that is currently being followed. The first was

Holmes’ classification, who suggested a first complete taxonomic system. He pro-

posed the order “virales” composing three suborders, namely, Phaginae (virus that

infects bacteria), Phytophaginae (virus that infects plants), and Zoophaginae (virus

that infects animals). He further created 13 families, 32 genera, and 248 species. Then

came the classification system that gained the community support known as the LHT

(Lwoff, Horne, and Tournier) system. This system grouped the viruses into one

phylum called “vira” with two subphyla defining the genetic material, which is either

DNA or RNA. This was further classified into classes based on the symmetry of the

viral capsids.

Later, an urgent need to have an official system for taxonomy led to the es-

tablishment of the International Committee on Taxonomy of Viruses (ICTV).

Thereafter, in 1971, David Baltimore published a classification system that is still in

use in parallel. He grouped all viruses into seven groups based on the type of

genome. From then until the present day, virus taxonomy has been considered by

this committee following the Baltimore classification system (Figure 2.1).

18

Bioprocessing of Viral Vaccines