You will undoubtedly already know that the SARS-coV-2 coronavirus, which causes the Covid-19 disease, has been a before and after for the recent history of humanity. Hundreds of thousands of people have died worldwide from this pathogen that has represented the largest global health emergency to date. However, surely you also know cases of people who have tested positive for Covid-19 and have barely had a slight cold or have not even had a single symptom. How is this possible? Why is it so lethal for some and for others just a cold?