A dog with no symptoms, whether nicely groomed and accompanying the right sort of owner, or not, can be carrying, in its fur, live eggs of
toxocara canis if it has not been treated which includes regular deworming. Humans can be infected by this roundworm, a condition called
toxocariasis, just by stroking an infected dog's fur and accidentally ingesting infective eggs that may be present on the dog's fur. When humans ingest infective eggs, diseases like
hepatomegaly,
myocarditis, respiratory failure and vision problems can result depending on where the larva are deposited in the body. In humans, this parasite usually grows in the back of the eye, which can result in blindness, or in the liver or lungs.
I wouldn't want to touch, or sit on, a seat on which someone's dog has been.