Knowledge is necessary to understand the world especially when it comes to medical issues. Knowing what we are up against helps us to react in a proper way and not to fear the threat. This is as important for researchers as it is for the public. One category of those threats are viruses.
What is a virus?
Credit: Graham Beards
A virus is an akaryocyte, meaning a cell without a nucleus. It is not a living organism (even though this specific question is still under a debate) in opposition with bacterias which they are often confused with.
It is composed with a nucleic acid that represent viral genome and can either be DNA (DeoxyriboNucleic Acid which contains the whole genetic information) or RNA (RiboNucleic Acid, similar to DNA, used to synthesise proteins) sometimes protected by a capsid or a viral envelop.
There are around 3,600 viruses described but only 129 are considered as infectious pathogens for humans. [1]
How does it live?
A virus requires a host to live, thrive and survive. This is mandatory for the virus to replicate themselves. It can have one of the two following life cycles: the lytic cycle or the lysogenic cycles.
A: lytic cycle; 1: attachment; 2: penetration; 3: transcription;
4: biosynthesis; 5: maturation; 6: lysis
B: lysogenic cycle; 1: attachment; 2: penetration; 7: transcription;
8: division; 9: spreading
Lytic cycle
The first one is a reproductive cycle (called lytic cycle) [2]:
– Attachment: the virus attaches itself to the surface of the host cell in order to inject its DNA into the cell
– Penetration: the virus injects its DNA into the host cell by penetrating through the cell membrane
– Transcription: the host cell’s DNA is degraded and the cell’s metabolism is directed to initiate virus biosynthesis
– Biosynthesis: the virus DNA replicates inside the cell, synthesising new virus DNA and proteins
– Maturation: the replicated material assembles into fully formed viruses
– Lysis: the newly formed viruses are released from the infected cell (which is itself destroyed in the process) to seek out new host cells to infect
To sum up, the virus infiltrates a cell, degrades it, replicates inside it, forms new viruses and destroys the cell to release the virus to infiltrate other cells.
Lysogenic cycle
The second one is a dormant cycle (called lysogenic cycle) [3]:
– Attachment: the virus attaches itself to the surface of the host cell in order to inject its DNA into the cell
– Penetration: the virus injects its DNA into the host cell by penetrating through the cell membrane
– Transcription: the virus’ genome incorporated use the cell’s genetic information to form a prophage
– Division: the prophage then remains dormant until the host cell divides
– Spreading: after the host cell has divided, the virus DNA in the daughter cells activate and begins to express itself. Some of the cells containing the prophage go on to create new viruses which will move on to infect other cells
To sum up, the virus infiltrates a cell, creates a dormant virus inside the cell waiting for the cell to split to spread.
A threat for others living creatures
When a virus replicates, it degrades the host cell and spread its viral genome which activate and overcome, highjack or inhibit normal cellular functions. It results in the death of the cell and the dissemination of the virus to other cells. The process will then repeat.
Viruses can infect animals, plants, bacterias, archaes and even other viruses. However, a specific virus can usually not infect every organism but target few species and not all cells of these species. Furthermore, the cell infected by the virus determines the symptoms associated to that virus. For example, influenza virus (a.k.a the flu) infects cells from the upper respiratory tract (nose, throat, bronchi) and sometimes lungs, giving coughs, soar throat, etc. Other symptoms coming from the reaction of the immune system, like fever.
A virus is characterised by two elements: its capacity to spread (pathogenicity) and its intensity (virulence).
How to protect ourselves
We have to be careful when we are dealing with viruses because since they use the cellular mechanics of their host to reproduce inside the cell, eliminating the virus often ends in killing the cell. However, several strategies exist.
Antiviral drugs
The first possibility is to use antiviral drugs to stop the replication of the virus. We use this treatment when the virus already infected an organism. Antiviral drugs inhibit the development of the virus.
The drug can intervene at different step of the virus replication. Before the entry in a cell with entry or uncoating inhibitor. During transcription, biosynthesis or any part of the viral synthesis. During the final phase, the release of the virus to other cells.
Another strategy is to use antiviral drugs to stimulate the immune system and let it fight the infection itself.
Vaccination
While antiviral drugs act when the virus is already in place, vaccination is a preventive measure.
The concept of vaccination is as follows: inject an inactive, harmless, modified version of the virus into the organism. It is spotted by our immune system as a threat to eliminate. When dealt with, the immune system will keep a memory of the virus and will have the agents to fight it more easily when an active one will enters our organism.
Since it is an inactive version, the reaction of the injection might not be enough to stimulate the immune system and make it act and allows stronger immune response [4]. This is why an element, call adjuvant, is added to provoke the reaction. The most common one is aluminium in its salt form and presents no increased risk of any serious reactions [5].
Vaccination also allows to fight an infection already in place, with less efficiency than preventive injection.
Treat the symptoms
When there is no solution, we treat symptoms, waiting for the virus to wear off and the immune system to deal with the problem.
The role in nature
If they are so harmful, why won’t we annihilate them all? Despite the impossibility of such a task, viruses have an important role in nature.
They are a vector in horizontal gene transfer (called HGT) between different individuals within the specie or even between different species. HGT increases genetic diversity (giving different tools to species to adapt to their environment which is a major characteristic of biodiversity) and spreads those genetic innovations further than just a specific individual and its lineage [6].
One current hypothesis is that viruses had a key role in evolutive history of all living things as we know them [7].
Viruses will always be a serious threat to living creatures such as ourselves but research is here to find the means to fight them.
References
[1] Human viruses table ~ ViralZone, https://viralzone.expasy.org/678
[2] Lytic cycle – Wikipedia, https://en.wikipedia.org/wiki/Lytic_cycle
[3] Lysogenic cycle – Wikipedia, https://en.wikipedia.org/wiki/Lysogenic_cycle
[4] Adjuvants help vaccines work better. | Vaccine Safety | CDC https://www.cdc.gov/vaccinesafety/concerns/adjuvants.html
[5] Jefferson T.; Rudin M.; Di Pietrantonj C. (2004), “Adverse events after immunisation with aluminium-containing DTP vaccines: systematic review of the evidence”, The lancet infectious diseases, Volume 4, Issue 2, Pages 84-90, February 01, 2004, https://doi.org/10.1016/S1473-3099(04)00927-2
[6] Canchaya C.; Fournous G.; Chibani-Chennoufi S.; Dillmann M-L.; Brüssow H. (2003), “Phage as agents of lateral gene transfer”, Current Opinion in Microbiology, Volume 6, Issue 4, August 2003, Pages 417-424, https://doi.org/10.1016/S1369-5274(03)00086-9
[7] Forterre P.; Philippe H. (1999), “The Last Universal Common Ancestor (LUCA), Simple or Complex?”, The Biologic Bulletin, Volume 196, Number 3, https://doi.org/10.2307/1542973