How to tell relatives not to feed baby from their mouth

Ee this is very disgusting. I think as a parent you can out rightly tell the person not to. If I'm not the parent I will still address it, cause it's too unhygienic. The person may not know what himself or herself is carrying and with the current virus outbreak, this shouldnt be tolerated. I know some aunties (including my mom) always do this to cool the food down when we were young. Even as a kid I already felt disgusted and was so traumatised that I dont allow my mom to feed my child now. This has to be stopped immediately.