According to the video, the places aren’t being built to be attractive to live in but to be cheap upfront investments for landlords (so not just small but crappy layouts and quality overall), and the majority of said landlords are “middle class” (their words) couples rather than large companies.
My first thought was misfolded prions. Sounds like one possible avenue of the potential harmful “environmental exposure” the microbiologist (and CJD researcher) in the article was talking about.