The lipid pneumonia outbreak was a thing exclusively associated with THC vapes, which are an illegal but widespread cottage (garage) industry where one summer, one of the thousands of manufacturer-enthusiasts made a forum post about the innovation of maybe using vitamin E acetate as a thickener. Experiments were performed, positive results were obtained, and products went out to distributors. The hazard to heavy users (perhaps for manufacturers with poor blending practices, we don't know) who showed up in the ER, was recognized within a month or two, and everybody immediately stopped using vitamin E acetate as a thickener. It took most of a year of panic for the last of that summer's merchandise to percolate through the supply chain.

The outbreak was initially hard for users to trace in particular because of how brands worked in that (again, moderately illegal) industry - a "brand" was basically a paper label/bag production line shipped in the clear from a printer, to hundreds of individual manufacturers, who negotiated their own distribution. Conclusions like "Mellow Mallow Blurple is a safe brand, I tested it" ended up being invalid.