Look at this thread: everyone has their “one weird trick” (substance) which they believe is the silver bullet. Including garlic.

I doubt that these people have financial stake in any of these products. Sometimes, the substance might be relevant to some opinion they hold dear. The guy recommending garlic might consider all modern medicine a conspiracy of the pharmaceutical industry. A significant number of people on HN and reddit have also been really into cytokines for the last few years, the reason for which eludes me (maybe it’s the one bit of biology they heard of at some point and they are still enamored by their smartitude and want everyone to take notice).

Quinine and similar might have just been popular for the funny Gin&Tonic tie-in. Then, the US president read about, called it “the biggest discovery in medicine” or similar on Twitter, and now the alt-right has a stake in its success.