What harm does eating on dishes and drinking from gles that have not been washed?

i wanted to know some information about how eating and drinking from dishes harms the body....would like to tell someone i know that just rinses their dishes and utensils after being used by another person. could anyone tell me any specifics on this. it to me has gotten too far when i notced a stinch coming from a cup they had just rinsed out and started drinking from it. please help!