What is COVID-19?

  • 1st Revision: Francesca Fitzgerald
  • 2nd Revision: Alex Jasnosz
  • 3rd Revision: Ha Nguyen

The virus causes symptoms by infecting healthy cells and making copies of itself by multiplying in them. The spiky surface proteins of the coronavirus allow for it to attach to the receptors present on the healthy lung cells.

Specifically, the viral proteins bust into cells through ACE2 receptors. Once inside, the coronavirus hijacks healthy cells and takes command. Eventually, it kills some of the healthy cells.

How Does Coronavirus Move Through Your Body?

Coronavirus spreads rapidly through droplets produced when an infected person coughs, sneezes, or breathes. These droplets remain in the air and on surfaces they  fell onto. If an uninfected person then touches their eyes, nose, or mouth, this creates a passage for the virus to enter a new host through the mucous membranes in the throat and nose.

Within 2-14 days of this happening, the immune system will respond and this can manifest in the following physical symptoms:

  • Fever
  • Shortness of breath or trouble breathing
  • A cough
  • Chills
  • Fatigue
  • Headache
  • Body aches
  • Congestion or a runny nose
  • A sore throat
  • Loss of smell
  • Loss of taste
  • Nausea or vomiting

The new coronavirus goes deeper into the body than a regular flu or cold, i.e. into the lower respiratory tract where more ACE2 receptors are present. That is why COVID-19 is more dangerous than the common cold.  

Your lungs might become inflamed, making it difficult for you to breathe. This can lead to pneumonia, an infection of the tiny air sacs (called alveoli) inside your lungs where your blood exchanges oxygen and carbon dioxide.

Although the symptoms end with a cough and a fever for most individuals, the infection can be more severe for others. Some patients may experience shortness of breath (dyspnoea) within 5-8 days of the infection or can have serious complications such as acute respiratory distress syndrome (ARDS).

COVID-19 can cause a range of breathing problems, from mild to critical. Older adults and people who have other health conditions like cancer, heart diseases, and diabetes may have more serious symptoms.

Coronavirus and your lungs

When the virus enters the body, it comes in contact with the mucus lining of the nose, eyes, and mouth. This leads the virus to a new healthy cell where it grows, multiplies and infects adjacent cells. For an easy explanation, let’s assume our respiratory tract is an upside-down tree.

The trachea or windpipe is the trunk of the tree which divides into smaller branches in the lungs. At the end of each branch, there are tiny air sacs called alveoli. Alveoli are responsible for the exchange of gases in the lungs i.e., oxygen goes into your blood and carbon dioxide comes out.

Both the upper and lower parts of the respiratory tract could get infected by a new coronavirus. The virus goes down the airways. The lining of the lungs can become irritated and inflamed. In fact, at times, the infection can reach the alveoli.

Recent research has shown that, while COVID-19 is a new condition, the effects on the body are related to Middle East respiratory syndrome (MERS) and severe acute respiratory syndrome (SARS), the two other coronavirus diseases that are known to have cause serious illness in humans1.

Mild and moderate cases

Most patients with COVID-19 experience mild to moderate symptoms with just a little dry cough and sore throat. However, some infected patients have suffered from a lung infection or pneumonia with the inflammation of the alveoli. This is due to the intense response of the immune system to an unfamiliar particle (the virus).

As soon as the virus enters, the lungs and airways start swelling and become inflamed (that is the innate, or primary immune response); our adaptive immune system then starts to fight back with specialised immune cells that may worsen the symptoms. As unpleasant as the symptoms are, they are important as they are a sign that the immune system is trying to fight the infection off.

Coronavirus and your immunity

The immune system starts attacking the virus cells as soon as the virus enters the body and tries to invade healthy cells. Innate immunity is the frontline defense system of the body that comes into action to overcome the intrusion of a virus. This response of our innate immunity is the initial reaction to infection.

Interferons (inflammatory proteins with antiviral functions) are then released as part of the initial response of the body. They are responsible for stopping the virus in the pathway itself.

However, according to researchers, the biggest challenge is that this new coronavirus can go undetected by our innate immune system. If that happens, the adaptive immune response cannot be activated. Without an immune response, the virus continues to infect healthy lung cells2.


1. Alqahtani, J.S., Aldhahir, A.M., Oyelade, T., Alghamdi, S.M. and Almamary, A.S. (2021). Smoking cessation during COVID-19: the top to-do list. npj Primary Care Respiratory Medicine, 31(1).

2. Di Mauro Gabriella, Cristina, S., Concetta, R., Francesco, R. and Annalisa, C. (2020). SARS-Cov-2 infection: Response of human immune system and possible implications for the rapid test and treatment. International Immunopharmacology, 84, p.106519.

This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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Anjula Gahlot

Master of Science, Global Public Health and Policy, Queen Mary University of London

Activities and societies: Elected as IFMSA Subcommittee member, Students for Global Health Society, Barts and the London School of Medicine and Dentistry; Active Member of St. Johns Ambulance Society.

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