What are viruses? A brief introduction to the wonderful world of Virology
Contrary to popular belief, the term 'virus' is not an all encompassing term for whatever makes us sick. Like bacteria, viruses are a distinct infectious agent able to cause diseases. Viruses can cause simple sickness like cough and colds to the more serious illnesses such as AIDS and poxes.
The Viral Structure
Viruses are very very small even in terms of microscopic organisms. They are around 100 times smaller than bacteria. You can't even see them under the a regular microscope, often times requiring an electron microscope just to visualize them! Because of their small size, viruses are very simple in terms of structure.
A virion (a single virus particle) is composed of:
- Nucleic Acid. In its very core, viruses contains a nucleic acid. A virus can have an RNA or a DNA but never both. This is arguably the most important part of the virus; it is involved in viral replication.
- Capsid. Protecting the viral nucleic acid and the outer covering of most viruses is the capsid. The capsid is made up of numerous protein capsules called capsomeres. The capsid comes in various shapes and morphologies. The most common are helical and icosahedral.
- Envelope. While some viruses only have the capsid for its outer layer, some viruses have an additional membrane called the envelope. The envelope is a part of the host cell that the virus infects as well as a few additional proteins courtesy of the virus itself. Although it may seem like it is only an additional layer, envelopes greatly contribute to a virus' infectivity.
How Viruses "Reproduce"
Despite being able to reproduce and evolve, viruses are not strictly considered to be 'living'. This is due to the fact that viruses do not have their own organelles (think human organs but for cells) and therefore cannot reproduce on their own. One of the requirements of life is to be able to reproduce using cellular division and viruses are just not capable of doing that. In order for them to reproduce, viruses need to infect a host cell and use that cell's organelles for its own purposes. In a way, viruses are like tiny little pirates that hijack a cell and use it for its own good!
There are several steps in viral replication namely:
- Adsorption. Adsorption or attachment is the first step in viral replication (we call it replication instead of reproduction because viruses can not reproduce, only make copies of themselves through the use of a host cell). In this phase, the virus attaches itself to cell it's targeting through the use of protein receptors and molecules.
- Penetration. While it may or may not be an intentional innuendo, the penetration phase of viral replication refers to the step in which the virus directly enters the cell. Non-enveloped viruses penetrate through directly going inside a cell through a process known as endocytosis. Enveloped viruses are a bit different though. Remember that the envelope came from the host cell itself? Enveloped viruses penetrate the host cell by merging its envelope with the outer membrane of the cell it's trying to infect.
- Uncoating. This is the step where the viral capsid is degraded by viral enzymes and proteases revealing the viral nucleic acid.
- Assembly and Maturation. After uncoating, the viral nucleic acid then proceeds to encode for and create the necessary proteins for new viruses via the infected cell's organelles. Once all proteins are created, the virus develops within the cell. During this time, the virus is undetected.
- Release. The newly assembled virus is released. Non-enveloped viruses destroy the infected cells in order to be released. In general, enveloped viruses will leave the cell through exocytosis, aka shedding.
Infection and Viruses
Viruses are infectious agents responsible for a lot of diseases like the common cold, the flu, measles and chicken pox, among others. Viruses are deadly too. They are responsible for most of the most serious diseases we have today such as AIDS, the Ebola virus disease, the avian flu, SARS, Hepatitis, etc. Some viruses can even cause cancer!
Despite the fact that viruses are deadly, there is quite a bit of difficulty in treating them. Unlike bacteria, antibiotics have absolutely no effect on viruses. This means that taking antibiotics in mild cases of flu is not only dangerous because of the possibility of bacteria to be resistant to them, it does absolutely nothing.
The most common treatment for viruses is our own bodies. The immune system does a very good job in removing the viral threat save for maybe the most serious of viruses. That's the reason why you're sent home to rest and drink plenty of fluids. You're leaving your body to do its job.
Prevention is also better than cure. While viruses don't have antibiotics, a preventive measure known as vaccines is available. Vaccines introduce non-infective parts of the virus to our immune system in order to prime it so that when the infection is present, our immune system will be able to act immediately. Despite the controversy surrounding vaccines, they are completely safe. Vaccines are responsible for the global eradication of the smallpox disease and as of now, has nearly eradicated Polio.
In the most severe cases, however, antiviral drugs are given. These types of drugs are DNA that the virus unwillingly uses in its replication process. The DNA insert then codes for a protein that has the capacity to stop viral replication.
The future in medicine and viruses are very bright. The more we understand viruses, the more we are capable of treating their various disease. One of the major concerns at the moment is in vaccine development. Several new vaccines have entered clinical trials and would hopefully be finalized soon.
Viruses are often used as delivery systems for therapy as well. This is useful in gene therapy and in developing stronger vaccines. Imagine a non-pathogenic virus carrying a vaccine for another virus!
Due to the rising amount of antibiotic resistant bacteria, viruses that infect bacteria has been looked at as well. The discovery of bacteria-infecting viruses has been around for a while and was only overshadowed by the discovery of the first antibiotic Penicillin. Quite the turn-around, no?