Several years ago I had a 9 year old Dane who hadn't had any shots in 2 1/2 years, I was going to do them all when her rabies came due. I worked at a shelter then, and brought home the most beautiful little brindle Dane pup you ever saw. She and the pup were together for two days when the pup developed a horrible case of the fast-acting parvo, and died within 24 hours.
I was so worried about the older dog being exposed, but the best advice/explanation I got was from a vet who told me that adult dogs who were vaccinated as pups and then go lots of places (my dog went everywhere except work) are the least at risk, and probably don't need anymore shots, the reason being that they are probably exposed to distemper and parvo at least once a month in their travels, and that exposure acts just like the shot does, keeps the antibody levels up.
She never did get sick at all, even though she was licking and playing with the pup right up until the time it got sick.