I decided to join Reddit and then send a PM (private message) to slugmaniac to try to get a solid reference to support his statement about PD progression speeding up during an infection such as the flu (see curem's link above).

We have now exchanged a few PMs, and what he has told me is that the evidence supporting that Reddit statement comes mainly from animal studies. Evidence from humans is largely anecdotal at this stage, but should strengthen as more clinical studies report their results.

He gave me a couple of links related to animal studies.

Systemic LPS Causes Chronic Neuroinflammation and Progressive Neurodegeneration

Neurodegeneration by activation of the microglial complement-phagosome pathway. - PubMed - NCBI

I think this issue is quite important for us PwPs, as it has a bearing on questions like "Should I get an annual flu shot?"

I haven't had the flu since childhood, but let's assume that I get the flu about once every 10 years. So over a ten-year period, what I am choosing between is ten short inflammatory responses verses one long inflammatory response. Which is better, in terms of increased PD progression? My gut instinct says that ten short inflammatory responses is better, but I suspect that nobody really knows for sure. Slugmaniac said that, while he tended to agree with me, a specialist immunologist would probably be the best person to answer this question.

But I guess this is actually a moot point, until the Reddit statement can be supported by more than anecdotal evidence and animal studies.

Curem, I hope that your father has now fully recovered from his bouts of flu and pneumonia. If that is the case, are you able to say whether you think that his PD has progressed significantly as a result of these infections?

Footnote: Slugmaniac has given me permission to share with the forum the information he has given me.