THE World Health Organisation says about 80% of people with Covid-19 recover without needing any specialist treatment.

During the first week, most people’s immune systems rally and manage to fight off the virus. Those who do not recover and continue to struggle for breath and have a fever often need to go to hospital.

At this more serious stage, the virus causes an inflammation in the lungs.

As the lungs’ functions deteriorate, the body responds by pouring out inflammatory material into the air sacs that are at the bottom of our lungs.

Lungs that become filled with inflammatory material are unable to get enough oxygen to the bloodstream, reducing the body’s ability to take in oxygen and get rid of carbon dioxide. At this stage, a patient may need to be given oxygen or put on a ventilator.

The most critical stage of the illness is when the body’s immune system goes into overdrive in what is called a cytokine storm. Cytokines are small proteins released by different cells in the body, including those of the immune system, where they coordinate the body’s response against infection and trigger inflammation.

Sometimes the body’s response to infection can go into overdrive and in its attempt to fight the virus, the immune system attacks the body’s own organs. The heart, the liver and kidneys are most likely to be affected and all of them will need to be supported by machines that can take over their function. The cytokine storm can lead to the patient’s death.

Cytokine storms are a common complication not only of Covid-19 and flu, but of other respiratory diseases caused by coronaviruses such as SARS and MERS. They are also associated with non-infectious diseases such as multiple sclerosis and pancreatitis.

The phenomenon became more widely known after the 2005 outbreak of the avian H5N1 influenza virus, also known as “bird flu”, when the high fatality rate was linked to a cytokine response. Cytokine storms might explain why some people have a severe reaction to coronaviruses. They could also be the reason why children are less affected, as their immune systems are less developed and so produce lower levels of inflammation-driving cytokines.

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