How are viruses and diseases classified, and what does this mean?

An early misconception with COVID-19  was that it was just 'like the flu' virus. The symptoms are alike but as we now know the diseases are completely different.  So how do we differentiate between viruses especially when the symptoms are similar?   

The truth is that viruses can be classified in a few ways:

1. Morphology-shapes and size 

People come in all shapes and sizes and apparently so do viruses. All viruses have a protein coat called capsids, made of building blocks called capsomeres. These capsids cover the genetic material (genome) of the virus.

Capsomeres can give viruses diversity in their shapes and sizes these include : 

• Icosahedral-Mulitple copies of capsid particles can arrange themselves in a helix or icosahedral shape.  The icosahedral shape has many subunits that form equilateral triangles that are arranged symmetrically. This shape allows the virus to have a stable shape and contain its genome. 

• Enveloped- Some viruses are surrounded, enveloped, by a lipid bilayer, the ones that do not have this layer are called naked viruses. The bilayer consists of glycosylated membrane proteins which can form peplomers. Peplomers which is glycoprotein spike like the one we see on pictures of COVID-19.

• Helical- helical capsids are made up of protomers (capsid proteins, that make up the capsomers)/ These wrap around a helical nucleic acid to form a helix. Helical capsids can be distinguished by length and number of protomer per helical turn. 
• Asymmetric (Complex)- These , as their names suggest, are not so straight forward and are a bit of everything. They can have traits of icosahedral and traits of helical shapes. They can also possess extra features such as protein tails. 
• Prolate - This is a variation of icosahedral shape it is found in bacteriophages.

2. Baltimore classification

This type of classification relies upon our understanding of the genetic material of the virus as well as the process to replicate it.  It describes the pathways to form mRNA as the diagram below suggests. 

Viruses are divided into 7 classes: 

• Class 1- These are made from double-stranded DNA, for example, smallpox or virus. 
• Class 2- Single-stranded (ssDNA) which forms a negative-sense strand which can be used as a template strand for positive-strand RNA and DNA synthesis. 
• Class 3- DsRNA where both parts of the genome are transcribed to produce mRNA.
• Class 4 - Positive (sense) ssRNA e.g. Rhinovirus that causes the common cold. 
• Class 5 - Negative (antisense) ssRNA which is complementary to mRNA so must be copied to a complementary positive-sense mRNA to make proteins using  RNA dependant RNA polymerase.
• Class 6 - These are positive and diploid strands. It can be used as a template for reverse transcription an example of these are retroviruses. 
• Class 7- These are double-stranded DNA viruses but have ssRNA intermediate in replication

Image: MicrobeOnline

3. Nucleic acids

Viruses only contain 1 type of nucleic acid.

All RNA viruses are single-stranded the exception is Reovirus, which is double-stranded. 

The ssRNA can be sense strand, where it can be used as mRNA (to be translated into protein) or antisense strand where it is complementary to the sense strand but can’t be used as mRNA.

4. Type of host Viruses are obligate intracellular parasites. This means that viruses are needy very needy in fact they completely rely on eukaryotic or prokaryotic cells for their replication  This is because viruses do not have the organelles and therefore able to replicate on their own, they must rely on a host organism Due to their range of host organism viruses can be classified according to their host organism.

Viruses that infect bacteria are termed to be bacteriophages. Bacteriophages have a complex shape to enable them to access the bacteria and therefore infect it. 

Diseases are named by the World Health Organisation using the International Classification of Disease as guidelines one being to use names that are easy to pronounce. Once the name of the disease is assigned it used globally to allow the disease to be recorded around the world. These are just a few ways viruses are classified and enable the scientific community to develop treatments that target the disease caused by that virus.