A virus (from the Latin word for toxin or poison) is a microscopic organism consisting of genetic material (RNA or DNA) surrounded by a protein, lipid (fat), or glycoprotein coat.
Some microbiologists classify viruses as microorganisms, while others don't because they are "non-living" and describe viruses as microscopic infective agents.
In this article, we will give an overview of viruses, how they evolved, an explanation of "friendly" viruses, and how we can defend against them.
Contents of this article:
Fast facts on viruses
Here are some key points about viruses. More detail and supporting information is in the main article.
What are viruses?
Viruses are one of the most populous biological entities on earth.
Almost every ecosystem on Earth contains viruses; they are considered the most abundant biological entity on the planet.
Before entering a cell, viruses exist in a form known as virions. During this phase they are roughly one-hundredth the size of a bacterium and consist of two or three distinct parts:
- Genetic material - either DNA or RNA
- Protein coat - called the capsid, this protects the genetic information
- Lipid envelope - sometimes present around the protein coat when the virus is outside of the cell
Viruses are unique microorganisms because they cannot reproduce without a host cell. After contacting a host cell, a virus will insert genetic material into the host and take over that host's functions.
The cell, now infected, continues to reproduce, but it reproduces more viral protein and genetic material instead of the usual cellular products. It is this process that earns viruses the classification of "parasite."
Viruses come in a wide array of shapes and sizes and are often fitted into the following categories:
- Helical - for instance, the tobacco mosaic virus.
- Icosahedral - these near-spherical viruses make up the majority of animal viruses.
- Envelope - some viruses cover themselves with a modified section of cell membrane, creating a protective lipid envelope. These include the influenza virus and HIV.
- Complex - a non-standard shape that can include both helical and icosahedral forms.
The origin of viruses
Because viruses do not leave fossil remains, they are difficult to trace through time. So-called paleovirologists instead use molecular techniques to compare the DNA and RNA of viruses to uncover more information regarding their origin.
There are three competing theories in this regard:
- Regressive hypothesis - viruses started as small cells that parasitized larger cells. Over time, they shed genes that did not assist in this parasitizing and became entirely dependent on the cells in which they inhabit.
- Cellular origin hypothesis - viruses evolved from sections of DNA or RNA that "escaped" from larger organism's genes.
- Coevolution hypothesis - viruses could have evolved from complex molecules of nucleic acid and proteins at the same time as the first cells appeared on Earth. This theory means that viruses would have been dependent on cellular life for billions of years.
What are "friendly" viruses?
Many of us will be familiar with friendly bacteria that exist in our intestines and help us digest food. Scientists from San Diego State University reported that humans also carry friendly viruses that help protect us from dangerous bacteria, including E. coli.
Jeremy Barr and his team believe that their discovery may change how several diseases are treated. They discovered that mucus contains bacteriophages - viruses that actively protect their hosts from harmful bacteria by destroying them.
"Taking previous research into consideration, we are able to propose the Bacteriophage Adherence to Mucus - or BAM - is a new model of immunity, which emphasizes the important role bacteriophage play in protecting the body from invading pathogens.
We envision BAM influencing the prevention and treatment of mucosal infections seen in the gut and lungs, having applications for phage therapy and even directly interacting with the human immune system."
How are viruses spread?
Viruses may spread vertically (from mother to child) or horizontally (from person to person). A virus' ability to spread depends on the makeup of the virus.
Some viruses can spread by simple contact, exchanges of saliva, coughing, or sneezing. Some require sexual contact, while others go through the fecal-oral route via contaminated food or water. Still other viruses require an insect like a mosquito to carry the virus from person to person.
What diseases are caused by viruses?
Influenza is caused by a virus.
Several human diseases are caused by viruses; these include:
- The common cold
- Human papilloma virus
- Hanta fever
- HIV (the virus that causes AIDS)
- Cold sores
- SARS (Severe acute respiratory syndrome)
- Epstein-Barr virus
- Some types of cancer
How do we fight viruses?
When the body's immune system detects a viral infection, it begins to respond. A process called RNA interference is started, this process degrades viral genetic material and enables cells to survive the infection.
The immune system also produces specific antibodies that are capable of binding to viruses, making them non-infectious. In addition, the body's T cells are sent to destroy the virus.
Although most viral infections result in a protective response from the immune system, viruses such as HIV specialize in evading the immune system using a number of different techniques.
Neurotropic viruses are also very capable of avoiding our natural immune system's response to infection. These viruses are capable of infecting nerve cells and are responsible for diseases that include polio, rabies, mumps, and measles.
How are viruses prevented and treated?
Whereas bacterial infections can be treated with antibiotics, viral infections require either vaccinations to prevent them in the first place or antiviral drugs to treat them; sometimes, supportive measures that just treat the symptoms of the infection are adequate.
Vaccinations are generally the cheapest and most effective way to prevent viruses. Currently, vaccinations exist for polio, measles, mumps, and rubella, among others. In fact, vaccinations have been instrumental in eliminating diseases, such as smallpox, and reducing other viral diseases to extremely rare status.
Virus vaccinations consist of a weakened form of the virus (live-attenuated viruses) or viral proteins called antigens that stimulate the body to form antibodies that will fight off future infections with the same virus. Live-attenuated vaccines carry the risk of causing the original disease in people with weak immune systems.
Antiviral drugs have been developed largely in response to the AIDS pandemic. These drugs do not destroy the pathogen but instead inhibit their development. There are also antivirals available for the treatment of infection with the herpes simplex virus, hepatitis B, hepatitis C, influenza, shingles, and chicken pox.
Viral infections tend to resolve on their own without treatment. While the infection is present, treatment usually focuses on symptom relief such as pain, fever, and cough.