What happens in the body when you test positive for Covid-19

Unlike other viruses – such as influenza or chickenpox – which generate approximately the same symptoms in all patients, Sars-CoV-2 has several faces.

In some people, it only triggers respiratory problems, while in others it causes digestive disorders, heart weakness, or even brain damage.

It is therefore not easy to navigate and predict in advance the progression of the disease in a given person. Several scenarios exist, which is particularly confusing for doctors and health services faced with the Covid-19 pandemic.

The nostrils and throat, the main entry points for the virus

Viral particles emitted into the air by an infected patient – by coughing, talking or simply exhaling – are particularly contagious. When a healthy person breathes them in, they start by attacking their respiratory system. They attach to the cells of the nasal and bronchial mucosa using proteins present in several copies on their envelope: the famous proteins Spike (tip in English), arranged in a crown, hence the name coronavirus.

As soon as they are firmly attached to their targets, these S proteins use specific enzymes (ACE2) like a Trojan horse to slip inside.

Once it has conquered territory, Sars-CoV-2 takes full power: it forces the cell to work for its benefit so that it makes several copies of itself. Thus replicated, the virus can attack other neighboring cells which it will in turn besiege and enslave. This chain process allows it to spread little by little in our body.

A French study from July 2021 also showed that Sars-CoV-2 destroys the cilia on the surface of the respiratory tract to slip more easily into the pulmonary alveoli. “The loss of cilia could limit the evacuation of viral particles produced locally, and therefore promote their progression deeper in the respiratory tree,” indicates Lisa Chakrabarti, of the Pasteur Institute.

Our immune system on alert

Faced with these increasingly invasive aggressors, our body's natural defense system goes on alert. To try to eradicate the intruders, it raises the body temperature, about a week after the start of the infection, which previously allowed Sars-CoV-2 to proliferate at will.

Other signs of Covid infection also appear due to inflammation caused by the viruses: headache, cough and nasal congestion following swelling of the mucous membranes. When the part of the nasal cavity involved in the perception of odors turns out to be very inflamed, a loss of smell (anosmia) can also occur, five to six days after the initial contamination.

Inflammation can reach high levels because the white blood cells of the immune system also release inflammatory molecules to call other defense cells to the rescue. If inflammation becomes severe, olfactory receptors and neurons deteriorate. The sense of smell then remains absent for several months, while the olfactory cells regenerate.

A response that varies depending on the individual

A second phase of the disease begins 7 to 14 days after infection. This differs greatly depending on the strength of the patient's immune system. If he has a strong immune system, the virus will be eliminated before it does too much additional damage. But if immunity is at half mast, due to treatment, a pre-existing pathology or advanced age, the virus spreads throughout the body, sometimes causing severe complications – pneumonia, secondary infection, respiratory distress, etc. – which require hospitalization.

The immune system can also adopt an inappropriate reaction to the situation: it goes into overdrive and causes the massive release of cytokines, inflammatory mediators whose excessive presence induces generalized inflammation. This is what scientists call the cytokine storm – or shock. 5% of deaths are due to this phenomenon, according to an American study published at the end of 2020 in Science Advances.

In the lungs, for example, this sudden influx of cytokine permeabilizes the small blood vessels in particular. As they begin to leak, the lung alveoli fill with fluid, which hampers breathing and causes acute respiratory distress syndrome.

Serious impact on vital organs

Contrary to popular belief, Covid is not just a respiratory infection. It attacks all organs of the body, starting with the intestine. This is why “the infection often causes digestive symptoms (diarrhea, vomiting, nausea, etc.) which can appear before respiratory signs and fever”, indicates Dr. William Berrebi, gastroenterologist and author of Microbiotic medicine (ed. Marabout).

The heart is not spared either. “20% of patients with Covid and 43% of those admitted to intensive care experience cardiovascular complications,” according to Dr. Vincent Aidan, cardiologist at Ambroise-Paré hospital in Boulogne-Billancourt. As cardiac cells also have ACE2 receptors, they are indeed easy prey for Sars-CoV-2. The state of acute inflammation caused by the infection is also conducive to myocarditis. The risk of heart attack is also increased because the disruption of blood circulation promotes the formation of clots capable of blocking the arteries.

The kidneys can also be affected. In intensive care units, kidney failure is a frequent complication of Covid, even in people who did not previously suffer from any pathology.

Brain integrity threatened

The brain is not spared from Sars-CoV-2 either, far from it. French researchers from the Brain Institute and the Pitié-Salpêtrière hospital have in fact identified traces of the virus – proteins and viral RNA – in the brains of patients who died from Covid-19. Their presence explains certain neurological complications (vertigo, encephalopathy, epileptic seizures, etc.), according to their study published in January 2021 in the journal Journal of Experimental Medicine.

Not content with attacking neurons directly, Sars-CoV-2 also damages the blood vessels of the blood-brain barrier, which nourishes and protects the brain from toxic substances. Result: the brain finds itself temporarily less well oxygenated and can be the site of microhemorrhages, likely to ultimately increase the risk of cognitive disorders and neurodegenerative diseases according to an October 2021 study from Lille University Hospital.