Back around 1966 I worked at a Summer retreat in the mountains.
My job was mainly running the milk shake machine. The first day I was there, the Sealtest man came by and showed me how to operate it. How to clean it with a safe disinfectant and also how to lubricate the O rings with again a safe grease.
He told me he could make the best milkshake I ever tasted but that it was illegal in North Carolina. I asked him why and he told me they were so rich they made people sick. Anyway he put in a couple of ice milk mix half gallons and soon made a milk shake. Sure enough it was the best one I had ever tasted. Also around 15 minutes later I began to get just a little nauseous.
At the end of the day, as instructed, I cleaned it, sterilized it and disassembled it until the next day. Maybe 30 years later I stopped by the same place and was surprised to see the same machine still there. I asked the guy operating it if he still cleaned it every day.
He told me he never cleaned it.
:-)
I just recall those old non-fastfood milkshakes as being very good. If I ever got sick, I never made connected it with the milkshake.