This document provides a brief introduction to viruses. It defines viruses as obligate intracellular parasites that can infect bacteria, protozoa, fungi, algae, plants, and animals. The document discusses some of the key discoveries in virology in the late 19th century. It also poses several questions about the nature and characteristics of viruses. The document then describes the basic properties of viruses, including their ultra-small size, lack of cells, protein capsids surrounding genetic material, and need to hijack host cell machinery to replicate. It discusses viral structure and different modes of viral multiplication.

1. A BRIEF INTRODUCTION OF VIRUSES
Concluded By-
Name- Musharraf Ali
Course- M.Tech (BCE)
IIT BHU
R.N.- 15012006

2. INTRODUCTION
• Viruses are obligate intracellular parasites of
bacteria, protozoa, fungi, algae, plants, and
animals.
• In the 1890, D. Ivanovski and M. Beijerinck showed
that a disease in tobacco was caused by a virus
(tobacco mosaic virus).
• Friedrich Loeffl er and Paul Frosch discovered an
animal virus that causes foot-and-mouth disease in
cattle.
• The concept of a filterable virus introduced at that
time.

3. • The exceptional and curious nature of viruses
prompts numerous questions, including-
1. How did viruses originate?
2. Are they organisms; that is, are they alive?
3. What are their distinctive biological
characteristics?
4. How can particles so small, simple, and seemingly
insignificant be capable of causing disease and
death?
5. What is the connection between viruses and
cancer?

4. Properties of Viruses
• Obligate intracellular parasites of bacteria, protozoa, fungi, algae, plants, and animals.
• Ultramicroscopic size, ranging from 20 nm up to 450 nm (diameter).
• Not cellular in nature; structure is very compact and economical.
• Inactive macromolecules outside the host cell and active only inside host cells.
• Basic structure consists of protein shell (capsid) surrounding nucleic acid core.
• Nucleic acid can be either DNA or RNA but not both.
• Nucleic acid can be double-stranded DNA, single-stranded DNA, single-stranded RNA, or double
stranded RNA.
• Molecules on virus surface impart high specificity for attachment to host cell.
• Multiply by taking control of host cell’s genetic material and regulating the synthesis and assembly
of new viruses.
• Lack enzymes for most metabolic processes.
• Lack machinery for synthesizing proteins.

5. The General Structure of Viruses
• Size Range-
Most of them are so minute(0.2μm) therefore an electron
microscope is necessary to detect them or to examine their
fine structure.
More than 2,000 bacterial viruses could fit into an average
bacterial cell, and more than 50 million
polioviruses could be accommodated by an average human
cell.
Animal viruses range in size from the small parvoviruses 1
(around 20 nm in diameter) to poxviruses 2 that are as
large as small bacteria (up to 450 nm in length).

8. Modes of Viral Multiplication

12. Techniques in Cultivating and
Identifying Animal Viruses
>Primary purposes of viral cultivation
-To isolate and identify viruses in clinical
specimens
-To prepare viruses for vaccines
-To do detailed research on viral structure,
multiplication cycles, genetics, and effects on host
cells

13. 1- Using Live Animal Inoculation
- Specially bred strains of white mice, rats,
hamsters, guinea pigs, and rabbits
- Occasionally invertebrates or nonhuman primates
are used
- Animal is exposed to the virus by injection
2- Using Bird Embryos
- Enclosed in an egg- nearly perfect conditions for
viral propagation
- Chicken, duck, and turkey are most common
- Egg is injected through the shell using sterile
techniques

14. 3- Using Cell (Tissue) Culture
Techniques
• Most viruses are propagated in some sort of cell
culture
• The cultures must be developed and maintained
• Animal cell cultures are grown in sterile chambers
with special media
• Cultured cells grow in the form of a monolayer
• Primary or continuous.