A virus is a small, non-living particle that invades cells of living organisms. Once in the cell, a virus uses the cell to make more of itself. A virus is considered a parasite - an organism that lives off of and harms another organism. The cell in which a virus lives is called the host.
Viruses are considered non-living because they do not use energy to grow or to respond to their surroundings. Likewise, they are incapable of reproducing on their own, i.e. without a host.
No organism is safe from viruses. Viruses infect organisms in all six kingdoms. However, most viruses are species-specific and cell-specific. That is, they can only infect certain cells in certain species.
When a human develops a cold, this is caused by a virus.
Viruses are not named like living organisms, i.e. using binomial nomenclature. Often, a virus is named after the disease it causes. Some viruses are named after the person(s) who discovered them.
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The Shape and Sizes of Viruses
Viruses vary in their shape and size. Below are some pictures of various viruses. Study them to see the various shapes.
Rabies Virus Rod-shaped Virus Herpes Virus Ebola Virus
Viruses are so small that they cannot be seen with the type of microscopes found in most high schools. Instead, scientists must use electron microscopes to view viruses. Viruses are much smaller than the cells they infect.
Why is it important that viruses are smaller than the cells they infect?
Viruses are smaller than the cells they infect because they have to be able to get into the cell. Plus, if viruses were too large, they would not be able to use the cell to reproduce themselves.
Structure of Viruses
All viruses have two basic parts: an outer coat (head) and an inner core.
The outer coat provides protection for the virus. The coat is made of proteins, and each virus has a unique protein that makes up its coat. The coat plays an important role in a virus' ability to attach to and infect its host cell. The coat is what determines what type of cells the virus is able to invade.
The inner core is made up of a virus' genetic material. It is this genetic material that holds the instructions for making more viruses.
The tail fibers assist the virus when it attaches itself to a host cell and inserts its genetic materials into the host cell.
Why would a virus want a protective outer coat?
Viruses are unwanted invaders to organisms' bodies. Therefore, organisms will try to destroy the viruses. The protective outer coat helps to protect the viruses so that they can infect the host cells and reproduce.
How Viruses Multiply
Viruses cannot reproduce on their own - they must have a host cell in order to reproduce. Click here to see the lifecycle of a typical virus. Scroll down until you see a red arrow. Click on the red arrow to view the steps of a virus' reproductive cycle.