It is not entirely wise to be rude and disrespectful to waiters and waitresses. First of all they are human beings like you and me, who have a very hard job as it is without having to deal with rude and obnoxious muppets like some of you.
Secondly, there is a window between when they receive the food from the kitchen and when they put it down on your plate with a plastic smile for your rude countenance. Anything can happen in that window including surreptitious introduction of various substances and/or bodily fluids.
Think about that tine next time you are rude and obnoxious to these hard working ladies and gents