Dog Saliva itself is not harmful to humans but if the dog has any parasites such as worms or any other disease then the disease or worms could be transmitted to the human through the saliva contact.
I've had dogs lick me in the face and even accidentally got dog saliva in my mouth and although gross I never got sick from it since the dog wasn't infected with anything.
Now I don't go around kissing a dog in the mouth or anything and I don't recommend anyone else do so. lol
But if the dog isn't infected with anything that you know of then simple contact with the dog saliva should not harm you.
Although you should use mouthwash to wash your mouth out if a dog does accidentally get some saliva in your mouth.