You are right that it takes a "bolus" of spores to contract an infection under normal conditions. You just about have to open the bottle and pour it into a sore!!!
However, they did feel that perhaps 3 spores into the lungs would cause an infection. Our bodies are counting on our nose hairs and mucus to catch them before the lungs.
However, I think you are thinking of smallpox as being one of the first vaccines........correct me if I am wrong.
Smallpox to cowpox