Virology an Introduction
Viruses are not biological organisms and are not classified in any kingdom of living things. They require a ‘host’ in order to replicate and spread. They infect cells by attaching to receptor molecules of the surface of their host cells, enter and hijack the cellular machinery of the living ‘host’ in order to replicate.
The ‘success’ of a virus lies in its ability to replicate and spread. Viruses have evolved over the centuries using different mechanisms to spread and multiply.
If a virus causes severe symptoms on infection, or kills its host, it is easy to catch and eradicate – like smallpox. If a virus can infect it’s host without immediately causing symptoms, it is difficult to catch & can spread widely – like HIV, HCV and now SARS-CoV-2.
SARS-CoV-2 infects adults and children alike, with different degrees of severity, and many of those infected never show symptoms. It is therefore a very ‘successful’ virus, and in order to stop the spread, widespread testing of the population is essential.
SARS-CoV-2 Terms & Statistics
SARS-CoV-2 Reproduction Number
Reproduction Number (R0) is a measure of how infectious the virus is.
If R0 is less than 1, each infected individual causes less than one new infection. This is the best case scenario as the disease will decline and eventually die out.
If R0 equals 1, each infected individual causes one new infection. This means that the disease will continue to be present in the population, but it won’t spread uncontrollably.
If R0 is more than 1, each infected individual causes more than one new infection, so with R0 = 2.5, every infected individual will spread to between 2 and 3 people, resulting in the current pandemic.
Infection Fatality Rate (RSA)
Infection Fatality Rate represents the percentage of deaths among all the infected individuals – including asymptomatic and undiagnosed. This number varies between countries.
This number published by the World Health Organisation means that 6 out of every 1000 people infected with
SARS-CoV-2, will die.
40% - 70%
The percentage of SARS-CoV-2 infections that are asymptomatic, or never experience symptoms, is uncertain. Current studies vary widely for a number of reasons including limited un-biased testing.
Broadly speaking if half the population is unknowingly asymptomatic, and every infected individual is likely to infect 2.5 people, we are very far from preventing the spread of COVID-19.
Infectiousness of asymptomatic individuals relative to symptomatic
Percentage of transmission occurring prior to symptom onset
Time from exposure to symptom onset
Time between symptom onset in an individual and symptom onset of a second person infected by that individual
How is it spread?
As with other flu’s the virus is spread mainly via respiratory droplets when an infected person sneezes, coughs, speaks or even breathes.
These tiny droplets remain in the air for some time.
The virus infects the body by attaching to cells in your nose or throat when you breath in these airborn droplets.
The virus can also stay ‘viable’ for some time on surfaces.
If you touch a surface that may have viral particles on it and then touch your face (nose, eyes or mouth) you can be infected.