The Biology of Covid-19

Advice from Dr. Nerina Harborne, PhD Biophysics, Biochemistry Lecturer, Mbarara University of Science and Technology.

The Covid-19 Pandemic, which started in Wuhan in China in December 2019, is caused by the SARS-CoV-2 virus. This is a mutant strain of the virus which caused the SARS pandemic of 2002-3.

A virus is a non-living collection of molecules that are packed up together, just like the sort of package we may send to each other.

The CONTENTS of the “package” are the instructions to make more viruses. These instructions are written in the language of RNA. This is the same language used by our own human cells to carry instructions. This means that our human cells can read the instructions and carry them out.

The WRAPPING on the “package” is made of lipid, or fat. The viral particles get these fats from the wrapping of our own human cells. The GOOD THING is that this wrapping is easily broken open, by detergents, such as we use in soap or washing powder, or by alcohol, such as is in hand sanitiser. Then the contents spill out, the address label falls off, and the viral package is destroyed!

The ADDRESS LABEL on the package is a protein (called “Spike Protein”) on the outside of the viral particle. This address label directs the package to a human cell and allows it to “deliver” its contents into the cell.

So, when the package is delivered, our human cells follow the instructions inside the package. The instructions tell them to make MORE viral packages, and then release them outside the cell, to be delivered to other cells!

All this is happening inside the cells of our nose and throat (upper respiratory tract).
So, when we cough or sneeze, all these little viral packages get caught up in the tiny water droplets that come out of our body (cough onto a mirror and you will see these tiny drops of water as they steam up your mirror!). These can hang around in the air for a short time, before slowly falling to the ground or other surfaces. 

So other people can become infected by breathing in the droplets with virus packages while they are in the air, or by picking up the ones that have fallen out of the air onto surfaces by touching them with their fingers, and then touching their mouth, nose or eyes and so getting the packages where they can be delivered to more human cells.

  • So – the advice to WASH YOUR HANDS WITH SOAP is to break open the packages and stop them from being delivered.
  • The advice to NOT TOUCH YOUR FACE is to keep the packages away from your own cells, which could follow the instructions to make more viral packages.
  • The advice to COVER YOUR FACE WHEN YOU COUGH OR SNEEZE is to stop water droplets carrying viral packages being delivered to someone else.
  • The advice to KEEP 2M DISTANT FROM OTHER PEOPLE is to make sure that any droplets you are breathing out do not reach the other person (and that theirs don’t reach you!).

Why are some people very ill with Covid-19, but others less so?

When the virus packages get inside a human body and are delivered to the cells inside the nose and throat, these cells start to follow the instructions inside the packages. They make more virus packages and put them outside the cells ready to be delivered.

Some of these packages get caught in water droplets, and are coughed, sneezed or breathed out, ready to be delivered and infect another human.

But some more of the packages stay inside the person already infected.

It’s what happens inside this person’s body that makes the difference to how ill the person becomes.

Our human bodies have a way of fighting off things from outside that can make a person ill. It’s called the Immune System, and it has LOTS of different weapons! The immune system can DETECT and DESTROY invaders.

So, if a person has a STRONG and HEALTHY immune system, it can use all its weapons to destroy the virus packages. The virus stays in the cells of the nose and throat and causes symptoms such as a sore throat and dry cough and a runny nose, a fever, and coughing and sneezing. Some people have such a strong immune system that they can destroy the virus packages before even getting these mild symptoms. However, these people with mild or no symptoms ARE STILL MAKING VIRUS PACKAGES AND BREATHING, COUGHING AND SNEEZING THEM OUT. They can still infect other people. Which is why we should ALL keep SOCIAL DISTANCING. We don’t know who is infected. We might be passing the virus to our friends. Our friends might be passing the virus to us.

Statistics show that about 80% – 8 out of every 10 – of infected people have mild or no symptoms. If you are young and strong, you will probably not become very ill if you are infected. BUT YOU MIGHT STILL PASS YOUR VIRUS PACKAGES ON TO OTHER PEOPLE WHO ARE NOT YOUNG OR STRONG.

In other people, the immune system is not so strong. People with weaker immune systems are often older people. Like so many parts of our bodies, the immune system gets tired and worn out after many years of life! People who have other medical problems also often have a weaker immune system, as it has used up a lot of its weapons in dealing with these other problems.

In these people, the virus packages are not destroyed quickly enough, and stay inside the body, being delivered to more and more cells. As the person breathes, the virus packages that were inside the nose and throat get carried down into the lungs.

In the lungs, the body continues to try to fight and destroy the virus packages. But in the lungs, the weapons that the immune system can use can also make it difficult for the person to breathe.

One weapon is to make sticky mucous (phlegm), to trap the virus packages. But this also sticks inside the lungs and makes it difficult for the air to get into the body.

Another weapon is for the body cells to let out lots of water, to try to wash away the virus packages. When this happens in the cells of the nose, it makes a runny nose. When it happens in the lung cells, the water stays in the lungs, rather like breathing in water and drowning.

So, the person with this infection feels very ill, and finds it difficult to breathe. They gasp for air, pulling lots of air into their bodies, but the oxygen in the air that our body needs can’t get past the mucous and water.

If this person is taken to hospital, the doctors will give them air with more oxygen (“put them on oxygen”). This helps the body to get enough oxygen by using the parts of the lungs which are still working.

If this is not enough, the doctors will use a machine to force oxygen into the lungs (“put them on a ventilator”).

Using oxygen or ventilators gives the ill person more time for their immune system to fight and destroy the virus packages. We have not yet discovered any other drugs or medicines that will help to destroy SARS-CoV-2, although many doctors and scientists are working very hard to find some.

Sadly, for some people, the battle is lost. Their lungs stop working completely before their immune system can destroy all the virus packages and stop them from damaging the lungs. This is what causes people to die from Covid-19 infection. This seems to be between 1 and 4% of infected people.


80% of people who are infected with SARS-CoV-2 virus will have MILD or NO symptoms of Covid-19 disease. But these people are still infectious and can give the disease to other people. 

16% of infected people will have SEVERE symptoms, of which the most important one is difficulty in breathing. Some of these people will need help from doctors in hospital to help them to breath.

4% of infected people will become CRITICALLY ILL. Some will die. These are mostly older people, or people with other medical conditions.

%d bloggers like this: