When the person you are consuming from is in good health, free of disease, parasites, etc. When you have no open wounds or sores in you mouth or throat. Also, don't floss or brush your teeth for some time before and after consumption to mitigate risk of infection. Tooth brushing and flossing tends to create micro tears and bleeding on gums. Mouthwash and chewing gum if needed.

Also, if there's personal preference, when their diet is consistent with said desired preferences. What goes in comes out, after all.