It’s hard to say who will die from virus

In every epidemic, some die, others become ill and recover, and the luckiest live through infection without symptoms. In today’s pandemic, we are seeing this play out before our eyes. Although the initial epidemiological data show that Covid-19 is more severe in older people, men and those with pre-existing conditions such as heart and lung disease, not everyone with severe disease has these risk factors. And not everyone at risk has the same symptoms, prognosis or outcome.
Why do people manifest such differences? And why is it not possible to predict an individual’s experience? To address this complex question, it is important to first get our terminology right. “Infection” means acquisition of the coronavirus after exposure to it. Infection is not synonymous with exposure — or with disease. Disease is a clinical state associated with cough, fever and other symptoms that ranges from mild to severe. These symptoms arise from damage to tissues and the immune system. Death
occurs when there is so much damage that the body cannot maintain blood oxygenation and other necessary functions.
In past epidemics, death and survival were attributed to providence or fortune. Modern medicine and science provide a better understanding of why infection can lead to such different outcomes. Among individuals in the same risk group — the same age, say — differences in infection outcome can result from five different variables outside their control.
The first of these is microbial dosage or inoculum, the number of viral particles that cause infection. Small numbers of viral particles are more likely to be contained effectively by the body’s defenses. Then, infection may cause no symptoms or only mild disease. In contrast, a large number of particles can lead to increased viral growth, overwhelming the immune system and causing more severe disease.
Genetics may also influence susceptibility to severe infection. Viruses often gain access to host cells via surface proteins, which vary in presence and nature from person to person. Someone with no such surface proteins may be resistant to infection. In the case of HIV, for example, some people lack the receptors needed for viral infection and are not susceptible to the virus.
A third variable that influences infection outcome is the route by which a virus enters the body. It’s possible that virus inhaled in the form of aerosolised droplets triggers different immune defenses than does virus acquired by touching contaminated surfaces and then touching one’s face.
—Bloomberg