Ebola. A variety of ebola was the biological agent in Tom Clancy' book
Executive Orders. The method of dispersal was aerosol cans set off in trade shos and airport terminals. The Jack Ryan character as president used an executive order to institute a national quarantine.
I have stockpiled canned goods and other supplies just in case. Even with smallpox I would imagine there will be some sort of quarantine until vaccinations are available.