Question: Why do some people have terrible symptoms whilst having COVID, and others don’t even know they’ve got it?

Valerie Vancollie answered on 10 Feb 2022:

This is because people’s immune systems are different, which happens for a wide variety of reasons. Everything from a person’s genetics, sex, age, diet, physical fitness, tiredness… can all have an effect. Which past illnesses you’ve had can also make a difference as scientists now think that some people have managed to use past exposure to other coronaviruses as a means of not getting Covid-19.

All of this combines to result in vastly different outcomes for people who get the same virus. A person’s susceptibility may also change over time because they could change their diet, physical fitness… and they will also age and the ability of the immune system to work properly does start to decrease from a certain age.

So there is a lot involved in who gets super ill and who might not even notice. This is why predicting how any one person will react is extremely difficult to impossible as there are too many factors involved.