COVID-19 is an infectious disease, but why are some people only mildly ill when we are all susceptible? And why do some people suffer from critical illness or even death? Is the virus smart enough to make its own choices? Or is it related to the patient's own immunity?

The Science ^[1] recently published an article on finding out why.

Who is more vulnerable?

• People with underlying diseases and the elderly are more likely to develop severe COVID-19: an analysis of nearly 45,000 confirmed cases in China found that the mortality of patients with cardiovascular diseases and diabetes were 10.5% and 7.3% respectively; those with chronic respiratory diseases, hypertension or cancer had a 6% mortality; those aged 80 years and older had the highest risk of death, with a 14.8% mortality rate.
• The male's mortality was higher than that of the female's: 2.8% in male and 1.7% in female, with a higher mortality in male patients. However, it remains uncertain whether there really exist gender differences, which must be observed among different populations in more different countries.

COVID-19 is more severe in these two groups of people, which may be related to the following factors:

• Usually, the body's immune system will play a role when 2019-nCoV enters the respiratory tract and infects and reproduces. At this time, the elderly and people with underlying diseases may have immune system dysfunction and immune cell overreaction, leading to pneumonia, shortness of breath and other diseases or symptoms in patients.
• The virus replicates faster than the immune system's response.

But these two points do not seem to explain why young people also die from COVID-19?

Angela Rasmussen, a virologist at the Mellman School of Public Health of Columbia University, believes that this may be linked to genetic and environmental factors such as genetic mutations, smoking or air quality.

Overall, all people are susceptible to COVID-19, but people with underlying diseases and the elderly are usually more severely ill, and some young people are also affected because of genetic and environmental factors.