Covid-19, what it is and how it behaves

Let's take a closer look at the new strain of the Coronavirus family.

by Luca Longo
29 April 2020
8 min read
by Luca Longo
29 April 2020
8 min read

What are viruses?

They don't breathe, they don't eat, they don't drink and they don't sleep. They don’t feel joy or pain. In short: they aren’t real life-forms. The first virus was identified in 1892, by Dmitri Ivanovsky: the Tobacco Mosaic virus. A few million different viruses have been discovered since then, although only 5,000 of these have been fully characterised.

Viruses have been found in the planet’s ecosystems: from the ocean floor to the tops of the highest mountains. If we take a litre of sea water, we can find 25 different types of viruses. In a kilogramme of marine sediment, there are around a million. This is possible because their sizes range from about 20nm in diameter (20 billionths of a metre) to a maximum of 250-300nm (the diameter of a human hair is about 300 times this!). Zoom out: it is estimated that there are about ten thousand billion, billion, billion individual viral units on Earth: 10³¹.  

What do viruses target?

Viruses do not all behave in the same way: each one specialises in infecting a specific living species, whether single-celled bacteria (i.e. made from a single cell), plants or animals. As they have developed, each of these has learned how to combat viral threats through their immune systems, but without succeeding completely: if weakened or hit by a large number of viruses and / or particularly dangerous viruses, it will not be able to cope with the attack or reject it effectively.

How does a virus behave?

Each virus is composed of an external shell, called a capsid, which contains the viral genome, a strand (single or double) of DNA or, more frequently, of RNA, which contains instructions for creating replicas of itself. This information is essential to enable the virus to reproduce, but it is not able to do it autonomously: it needs to infect a host organism, reprogramme it and force it to produce copies of the virus.

How does a virus reproduce?

It starts with a mechanism similar to linking two pieces of a puzzle: the virus capsid has protein structures, tangled into characteristic shapes, which interlock perfectly with structures present on the surface of the compatible host cells. When the two pieces fit together, the virus manages to attach itself to the cell, opens a gap in its cell membrane and injects its viral genome inside. Once inside, it "deceives" the host cell and induces its reproductive system to produce new copies of the capsid and viral proteins, creating a complete virus that can “break free”, escape from the cell (killing or damaging it) and go on to attack the neighbouring cell, in a chain reaction.

What are coronaviruses and how are they made?

Coronaviruses are a family of viruses that specialise in attacking mammals (including humans, albeit less frequently) and birds. The genome is protected within a tiny shell, it consists of a single strand of positive polarity RNA (ssRNA +), which is very large compared to other RNA viruses, as it has a sequence ranging from 26 to 32 thousand nucleotide bases (the "building blocks" of the genetic code). This codes for 7 different viral proteins.
Going from the inside to the outside, the RNA is covered by a nucleocapsid, a layer consisting of the M protein or membrane and an envelope protein, composed of phospholipid bilayer which is derived from the infected cells. It’s this last layer that "deceives" the immune defences of host cells, which don’t recognise it as an external threat. Finally, there are hammer-shaped tips (called spike proteins) on the surface of the capsid. These are the S glycoproteins that cross the envelope protein and act like harpoons to attach to the infected cells and give the virus its characteristic crown shape.

How big are coronaviruses?

Coronaviruses have a diameter of 80-160 nanometres (nm). About a thousand would need to be lined up to equal the diameter of a human hair: about 70 micrometres (µm), that is 70,000 nm. As a comparison, the fine powders – known as PM10 - have a diameter of 10 µm, that is 10,000 nm.

What harm can coronaviruses do?

In humans, coronaviruses can cause respiratory system infections, with damage of varying severity depending on the virus in question: they range from mild or moderate upper respiratory tract diseases, like the common cold, to pneumonia, severe acute respiratory syndrome, kidney failure and even death. To date, there are seven known human coronaviruses, which are common around the world. The most common - and least harmful - ones were discovered in the 1960s and cause a common cold. Other, more dangerous, ones were identified in the early 2000s and cause more serious respiratory tract infections. These include SARS-CoV, identified in 2003 and which causes Acute Severe Respiratory Syndrome (SARS), which appeared in China in November 2002 or the Novel Coronavirus 2012 (2012-nCoV) which causes the Middle Eastern Respiratory Syndrome from Coronavirus (MERS ), which exploded in Saudi Arabia in June 2012.


What is SARS-CoV-2?

On 31 December 2019 a new coronavirus strain was reported in Wuhan, China. It was identified as a new Coronavirus ß-CoV beta strain of the 2B Group, with 70% genetic similarity to SARS-CoV, the virus responsible for SARS. In the first half of February, the International Committee on Taxonomy of Viruses (ICTV), in charge of the designation and naming of viruses (i.e. species, genus, family, etc.), named the new coronavirus: "Acute severe respiratory syndrome coronavirus 2" (SARS-CoV-2).

What is COVID-19?

Coronavirus SARS-CoV-2 attacks the respiratory system of humans (and of some animals, although statistically this is less significant) and is able to develop the Coronavirus Pathology 2019, known as COronaVIrus Disease 2019 or COVID-19. The spike proteins in the virus are compatible to link together with the proteins found in the human epithelial cells of the respiratory tract and, more precisely, in the epithelial cells of the mucous membrane and in the alveolar lung tissue. This is why the virus affects the cells of the lungs and alveoli.

How is SARS-CoV-2 transmitted?

Based on the data currently available, the World Health Organization (WHO) states that contact with symptomatic cases (people who have contracted the infection and have already manifested the symptoms of the disease) is the main driver of transmission of the new coronavirus SARS -CoV-2.

The WHO is aware of a possible transmission of the virus from people who are infected but still asymptomatic (approximately two days before the onset of symptoms), although it highlights that this is rare and therefore has low infection incidence.

SARS-CoV2 never travels alone, but always in an aqueous environment. Evidence shows that transmission between humans mainly occurs through the passage of saliva droplets containing the virus, from an infected individual to the respiratory system of a healthy individual.

The transmission can be direct (contact with saliva, through a cough or a sneeze) or via the hands (when contaminated hands touch the mouth, nose or eyes). It is not yet clear how frequent surface-mediated transmission is (for example, by touching an object immediately after a sick individual has left a trace of moisture containing the virus on it). In rare cases, contagion can occur through faecal contamination.

To avoid infection, it is therefore essential to wash and disinfect your hands frequently and thoroughly with soap and water, for at least 20 seconds (preferably 40-60), or alternatively with an alcohol-based hand disinfectant containing at least 70% alcohol.

How is a coronavirus infection treated?

There are no specific treatments for infections caused by common coronaviruses. In fact, most infected people recover spontaneously. There are currently no specific therapies for treating the new coronavirus SARS-CoV-2: the symptoms of the disease are treated (this is known as supportive therapy) in order to promote healing, for example by providing respiratory support. As COVID-19 is a new disease, vaccines are not yet available to protect against the virus. The time needed to create an ad hoc one can be relatively long (an average of 12-18 months is estimated).

Where can we find information to understand how to protect ourselves from coronaviruses?

All the information contained in this Q&A comes from official sources, primarily the World Health Organization (WHO), which is the world's largest body for the study of diseases affecting mankind.