Viruses are non-living particles that contain genetic material and infect living host cells. They have some properties of life like being able to be killed but lack others like homeostasis. Viruses come in a variety of shapes and sizes measured in nanometers. They are made up of a protein coat and nucleic acid core and can only reproduce inside a living host cell. Important discoveries about viruses include identifying that they cause disease, crystallizing the tobacco mosaic virus, and developing the smallpox vaccine.

2. Viruses:
• Viruses are both and neither living or non-living.
• They have some properties of life but not others
For example, viruses can be killed, even crystallized
like table salt.
• However, they can’t maintain a constant internal state
(homeostasis).
• Def:Def:
A virus is a non-cellular particle made up of
genetic material and protein that can invade living
cells.

3. Discovery:
•Beijerinck (1897) coined the Latin name “virus” meaning
poison.
•He studied filtered plant juices & found they caused
healthy plants to become sick.
•Wendell Stanley (1935) crystallized sap from sick
tobacco plants.
•He discovered viruses were made of nucleic acid and
protein.
•Edward Jenner (1796) developed a smallpox vaccine using
milder cowpox viruses.
•Deadly viruses are said to be virulent.
•Smallpox has been eradicated in the world today.

4. Viewing Viruses:
Viruses areViruses are smaller than the smallest cellsmaller than the smallest cell
Measured inMeasured in nanometersnanometers
Viruses couldn’t be seen until theViruses couldn’t be seen until the electronelectron
microscopemicroscope was invented in thewas invented in the 2020thth
centurycentury

5. Characteristics:Characteristics:
•Non living structures
•Noncellular
•Contain a protein coat called the capsid
•Have a nucleic acid core containing DNA or RNA
•Capable of reproducing only when inside a HOST cell
•Some viruses are enclosed in an protective envelope
•Some viruses may have spikes to help attach to the host cell
•Most viruses infect only SPECIFIC host cells

6. Characteristics:Characteristics:
•Viral Capsids (coats) are made of individual protein subunits
•Individual subunits are called capsomeres
•Outside of host cells, viruses are inactive
•Lack ribosomes and enzymes needed for metabolism
•Use the raw materials and enzymes of the host cell to be
able to reproduce

7. SHAPE:
• Viruses come in a variety of shapes
• Some may be helicalhelical shape like the Ebola virus
• Some may be polyhedralpolyhedral shapes like the influenza
virus
• Others have more complex shapescomplex shapes like
bacteriophages

8. Viral Taxonomy:
1. Family names end in – viridae
2. Genus names end in - virus
3. Viral species: A group of viruses sharing the
same genetic information and ecological
niche (host).
4. Common names are used for species
5. Subspecies are designated by a number

9. Examples:
1. Lentivirus
2. Herpesvirus
3. Retroviridae
4. Herpesviridae
5. Human herpes virus 1, HHV 2, HHV 3
6. Human Immunodeficiency Virus 1, HIV 2