The size of a virus about 30-300 nm and its only visible with an electron microscope.  It has a

protein coat around the DNA or RNA sometimes has spikes. It has no cell structures.

How a virus multiplies

1. Virus ejects its DNA or RNA into the cell

2. The genetic material multiplies

3. Mew viruses are formed inside the cell and then burst out of the cell.

A virus is an extremely small fragment of nucleic acid (DNA or RNA) surrounded by a protein coat. It is smaller than a living cell. A virus is not a cell. The study of viruses is called Virology. Examples of viruses are shown in Figure 11.1. 

Characteristics of viruses 

(a) They do not have a nucleus, cytoplasm, or cell organelles. 

(b) They have a simple structure consisting of a small piece of nucleic acid (DNA or RNA) surrounded by a protein coat called a capsid. Some viruses have viral envelopes. These are membranes enclosing the capsids. The envelopes are made up of proteins from the host cell. 

(c) They cannot reproduce on their own. They must attack a host cell and use the materials in that cell to reproduce. This is called obligate parasitism. The ability of a virus to reproduce inside the cell and crystallize in the absence of a living host places them between living and non-living things. 

(d) When outside a host cell, they show no symptoms of life ( do not grow, feed, excrete, or respire). They exist in a dormant state.

 (e) They are host-specific. This means that a certain type of virus only attacks a specific host. For example, the rabies virus affects only mammals and HIV attacks only certain types of white blood cells in human beings. 

(f) Most viruses are infectious: This means they can cause diseases to their hosts. 

The structure of viruses 

Viruses are composed of strands of genetic material (DNA or RNA) which forms the core. The core is enclosed by a protein coat called capsid as in bacteriophage. DNA or RNA are of various shapes and sizes according to the type of the virus. 

Examples of the structures of viruses are shown in Figure 11.3. 

Advantages of viruses  

Although most viruses cause diseases, they also have the following advantages:

 (a) Viruses are important in the study of cellular and molecular Biology. They are used by scientists to manipulate and investigate the functions of cells.

(b) Some viruses are used to make vaccines. For example, the first vaccine against smallpox was a small dose of the virus that causes cowpox, which is a mild infection. On recovering from cowpox, the body had antibodies that could resist both cowpox and smallpox. 

(c) Bacteriophages are viruses that attack bacteria. They help in controlling bacterial infections and diseases. 

(d) Some viruses are used in biological control to eradicate pests such as insects. 

Disadvantages of viruses 

(a) Most viruses are pathogenic. They cause diseases and infections such as tobacco mosaic disease, tomato spotted wilt disease, cassava mosaic disease, rabies, chickenpox, COVID-19, polio, and AIDS. 

(b) Pathogenic viruses can reproduce very fast, leading to large-scale epidemics. 

(c) Viruses can alter themselves often, and thus become difficult to cure viral infections. For example, there are many different types of viruses that can cause common cold and influenza. This makes it difficult to develop vaccines or cures for these infections