Viruses are relatively simple organisms (although it can be debated as to whether they are considered a living organims or not). They consist of nothing more than a protein coat and nucleic acid (DNA or RNA). Viruses are significantly smaller than most organisms, even bacteria at around 20 nm in diameter. They are not visible through a light microscope. Some common viruses are: Adenoviruses, Influenza and Polio.

Basic Structure of Viruses

The outer protein coat of a virus is known as a capsid.
The capsid of a virus can come in a variety of shapes: rod, polyhedral or complex.
Examples of different viral shapes. Some viruses have an additional coating called a viral envelope which helps it attach to the membranes of the host cells they infect. The capsid and the viral envelope are also the part of the virus that is most prone to mutation. This is advantageous for the virus because it helps the virus stay undetected longer by a host's immune response.

Viruses are a form of a parasite that infects a variety of host cells. Each virus has a host range of cells that it can infect. For example: a Tobacco Mosaic Virus can only infect plants, not humans. The host range defines the types of cells that viruses attack. All viruses get into the genome of the host cell by injecting their genetic material. Once the genetic material is inside, it takes over the cell and forces it to manufacture viral particles. This continues until the host cell is so full with viruses that it bursts.

There are two main cycles of viral infection, the lytic cycle of viral infection and the lysogenic cycle of viral infection.

In the lytic cycle of viral infection the virus uses tail fibers or the viral envelope to attach to the outer surface of the virus. The virus then will inject its nucleic acid (DNA or RNA) into the host. The empty capsid of the virus will fall off the host cell. The viral nucleic acid will then by hydrolyzed, or cut into pieces. The virus can then force the host cell to begin manufacturing of new viral particles. It will use the enzymes and machinery of the host cell to accomplish this as well as to assemble the new viruses. As the new viruses are assembled, the host cell will swell full of viruses. The cell will eventual lyse or burst when it cannot hold any more viruses. These new viruses can then go on to repeat the process with new cells. The lysogenic cycle of viral infection is similar to the lytic cycle, but adds an additional mechanism that helps it survive longer in the host. In the lysogenic cycle the viruses can inject their genetic material with one paramount difference: it forms something called a prophage. A prophage forms when the viral genetic material integrates itself into the genome of the host cell without disturbing it. This is a great advantage to the virus because the viral genetic material will be replicated right along with the host genome. The prophage is undetected by the host's immune system or defense mechanisms. As a result many cells are created by the host that unknowingly has viral genetic information. At any given time, the prophage can trigger viral replication which moves that particular cell into a lysogenic cycle. Even though many cells may have prophages, they do not usually all jump into the lysogenic cycle at the same time. It is for this reason that the host will never be rid of a lysogenic virus once they are infected. They will have the virus for life. Examples of lysogenic viruses include are Herpes I and II.

The modern viruses of today are also more complex than just lytic or lysogenic viruses. HIV, or Human Immunodeficiency Virus is an example of a retrovirus. A retrovirus uses RNA as its genetic material and is extremely complex in its reproductive methods. A retrovirus has an enzyme called reverse transcriptase. This enzyme can force the host to transcribe the RNA of the virus into a DNA template of viral genome. Then the host cell will become a manufacturing center for viral particles that are transported via vesicles out of the cell. The host cell will continue to function without lysing. It has now become a virtual manufacturing plant for the virus.

New viruses are constantly emerging and mutating in our world. The host range of a virus that can change if two hosts are closely related. For example, swine flu has emerged as the result of humans working near pigs in the meat industry. The virus mutated and became compatible with humans. Other viruses are emerging that are far more deadly. Ebola Zaire is one of the most deadly filoviruses on Earth. Ebola replicates so quickly that it can destroy a host within 72 hours after infection.
Artistic depiction of an Ebola virion
Other viruses are becoming more prevalent again as some children are not being vaccinated for viral diseases. Smallpox, Mumps, Measles And Pertussis have all made a come-back in the last five years.

Some viruses are capable of infecting bacteria and are known as bacteriophages.

Structure of a bacteriophage.