So, let’s think about the measles. That’s a very good example because it’s a strong infectious disease, and it has a high fever, and then you get over it in a week… and I’m looking at that process, what’s involved in that. That’s a concerted outpouring of the entire immune system, the result of which is that the measles virus is expelled from the blood… all of these mechanisms are activated. And as a result of that, what I would call natural immunity, which is life-long, it’s absolute. It’s qualitative immunity, that is to say, first of all it’s specific… I will never again get the measles, no matter how many times I’m re-exposed to it. But what we often don’t think about is there’s an important non-specific immunity, namely I have primed the immune system to respond in this acute, vigorous manner, and so it’s ready to go for whatever is going to come down the pike in the future.