The immune system’s response to the same disease that it has already faced will be both faster and stronger.  The fact that it’s faster makes sense, since it already has the successful antibody.  However, the explanation for the fact that it’s stronger is that it’s a mutation due to random chance.  Since the memory response is pretty much invariably stronger, I’m not really buying that it’s “random chance”.  It just sounds like a “yeah, we totally understand this completely” thing.  It sounds like it is a mutation that just isn’t completely understood yet.