But that's the case for literally every infectious disease. We all have slightly (to dramatically) different immune systems and other systems. Even without being immunocompromised, some people die from seemingly innocuous diseases while others survive the most deadly diseases there are.

Even Black Death killed "only" an estimated 50% of the population. That means that there were necessarily a lot of people that got infected and survived it, probably some of them asymptomatically. Bodies are complex and varied, and there's no 100% way to predict how they'll respond to any given situation.