Small home flocks are far less susceptible to that kind of thing. They mostly spread in large operations where health is a secondary concern for the birds. Ducks aren't usually vaccinated, and don't need to be. Their natural internal temperature is feverishly high by most animal's standards, and so very few pathogens can live inside them. When they do get sick, it's usually (not always, but usually) due to mismanagement--poor diet, poor hygiene, etc.