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What is a Virus?

Introduction
First, let me tell you what a virus is NOT. A virus is not a bacterium, nor an independently-living organism. A virus cannot survive in the absence of a living cell within which to synthesize copies of itself (replicate). Antibiotics do not harm a virus; it is for this reason that treatment for the "flu" for example, is mainly to help ease the symptoms of the illness rather than to kill the organism which causes the "flu" (Influenza virus).

Well then, what IS it?
Now, is there a simple explanation which can define what a virus IS? Hmmmm... that's actually a tough question. A virus is not strictly alive.. nor is it strictly dead... A virus has some fundamental information (genes made of DNA or RNA) which allows it to make copies of itself. However, the virus must be inside a living cell of some kind before the information can be used. In fact, the information won't be made available unless the virus enters a living cell. It is this entrance of a virus into a cell which is called a viral infection. Too, the virus is very, very small relative to the size of a living cell. Therefore, the information the virus can carry is actually not enough to allow it to make copies (replicate). The virus uses the cell's machinery and some of the cell's enzymes to generate virus parts which are later assembled into thousands of new, mature, infectious virus which can leave the cell to infect other cells. Poliomyelitis virus for example, may have over one million copies of its basic genetic information (RNA) inside a single, infected human intestinal mucosal cell.

What does a virus look like?
Moving from the outside to the inside, here are some parts of a virus which are common to many different kinds of viruses: capsid, core, genetic material (DNA or RNA). The capsid is the outer shell of the virus which encloses the genetic material within. The capsid is actually made of many, many identical individual proteins which are assembled very precisely to form the capsid structure. Sometimes there will be a protein core underneath the capsid which also surrounds the genetic material. Some viruses may have an additional covering on the outside called an envelope. An envelope is kind of like skin around the outside of the virus. The envelope is actually a lipid bilayer (membrane) with proteins embedded within the membrane. If you examine a baseball, take it apart, you will see how some viruses are assembled. The cover of the baseball (envelope), the tightly-woven thread (capsid), and the rubber core (genetic material) can be used to represent the parts of some viruses.

What do viruses actually do?
All viruses only exist and make more viruses. And with the possible exception of bacterial viruses which kill harmful bacteria, all viruses appear to be harmful because their replication leads to the death of the cell which the virus entered. A virus enters a cell by first attaching to a specific structure on the cell's surface via a specific structure on the virus surface. Depending on the virus, either the entire virus enters the cell, or perhaps only the genetic material of the virus is injected into the cell. In either case however, the ultimate result of viral infection is the exposure of virus genetic material inside the entered cell. Then, the virus material essentially "takes over" the cell and nothing but viral parts are made, which assemble into many complete viruses. These viruses are mature and leave the cell either by a process called "budding" (just one or a few viruses at a time leave the cell) or by a process called lysis (the cellular membrane ruptures and releases all of the virus particles at once).

What things can become infected by a virus?
So far, there is not a living thing identified that doesn't have some sort of susceptibility to a particular virus. Plants, animals, bacteria - every living thing, whether multicellular or single-celled, can be infected with a virus specific for the organism. And, within a species, there may be 100 or more different viruses which can infect that species alone. So, whenever viruses are discussed, they are discussed as being either plant, animal or bacterial viruses - which means that an animal virus only infects a certain animal, and a plant virus only infects a particular plant. We say that a virus is specific for a particular thing if the virus infects only that thing. So, there are viruses which infect only humans (smallpox), some which infect humans and one or two additional kinds of animals (influenza), some which infect only a particular kind of plant (tobacco mosaic virus), and some which infect only a particular species of bacteria (lambda bacteriophage which infects E. coli).