Like I said, I can't find the study right now, and I originally saw the data anywhere between 5-10 years ago. They may also have been measuring some cytokine. Even without AD, memories really aren't reliable over that time span.

The point the authors were trying to make is that if you have AD, any inflammation, even asymptomatic ones, will potentially worsen your cognitive performance. The most interesting feature of that graph was to me that the cognitive performance actually remained fairly constant in between infections/inflammations, and decline was not gradual but in a step-wise fashion.