How Covid-19 kills: Understanding the spread of coronavirus in human body

Coronavirus starts from the upper respiratory system, and then moves to lungs filling them with fluid and causing pneumonia.

Coronavirus first originated in China in December last year, but has spread to more than 100 countries and infected over one lakh people. It has also led to the death of more than 3,000 people.

A passenger goes through a health screening after leaving a cruise ship in Oakland, California, on March 11. (AP Photo)

Tracking Coronavirus Outbreak: Track Live Updates

Covid-19, the disease caused by coronavirus, is known to kill between 3-4 per cent of those it infects.

“Covid-19 is not very fatal, but it is very infectious. Since it infects more people, a large number die. In terms of infectivity, it is almost up there with the most infectious measles and chicken pox... We don’t really understand the reason for it right now. However, we know that it moves from the respiratory system to gastro-intestinal tract, to the blood, leading to multi-organ failure and death,” said Dr Shobha Broor, former head of microbiology department at All India Institute of Medical Sciences (AIIMS).

The Sars-Cov-2 virus moves from the back of the throat, to the lungs and then the blood, according to an analysis of the disease progression in 191 patients from Wuhan published in the journal “Lancet” on Wednesday.

Once inside the body, the virus uses a spike-like protein to bind with a receptor called angiotensin-converting enzyme 2 (ACE-2) found on the respiratory cells, entering it and replicating to spread the infection within.

As the virus keeps multiplying it reaches the lungs, causing inflammation in the alveoli or lung sacs filling them fluid and pus causing pneumonia. The inflamed lungs make it difficult for the person to breathe leading to acute respiratory distress syndrome between day eight and day 15 of the onset of symptoms.

The recent study of the 191 adults admitted to Jinyintan and Wuhan Pulmonary Hospitals in China shows that 48 per cent had a co-morbid condition - hypertension being the most common (30 per cent), followed by diabetes (19 per cent), and coronary heart disease (8 per cent).

“Although the virus is around 70 per cent similar to the one that caused Sars in 2003, it is not a very good similarity. This virus is closer to bat coronavirus. But, we can infer from the way Sars patients dies that even in this infection a cytokine storm might be leading to death,” said Dr Broor.

Cytokine storm is an over-production of immune cells in response to an infection that can result in damaging the body tissues and organs.