> some people would win that race, and some wouldn't. You've basically answered your own question. At the population level, immunity is probabilistic. At the i

> some people would win that race, and some wouldn't. You've basically answered your own question. At the population level, immunity is probabilistic. At the individual level, it's ~deterministic. Those who are challenged but stay healthy are immune. Those who are challenged but get sick are not immune. If we were to repeat the experiment, the individuals who stay healthy/get sick would largely remain the same. In this ex-post view of things, there's no "in-between." You got sick or you didn't, you're immune or you're not. From an ex-ante perspective, we would instead try to predict whether a person will get sick or not. The answer to this largely depends on whether the body can generate a response to previously recognized antigens. This again is binary. You have memory B-cells due to primary response or you don't; you're immune or you're not.