Biological agents usually are dispersed by an aerosol spray that must be either inhaled or ingested. Biological agents usually have an incubation period of several days before any symptoms appear. Chemical agents are absorbed through the skin or by inhalation. Chemical agents typically cause immediate symptoms. The Center for Disease Control has a section on infectious diseases with detailed information, including maps, images and fact sheets.
Anthrax is a bacteria.
It has a spore form that makes it extremely resistant to the environment. It is highly infectious and lethal when inhaled. An anthrax vaccine does exist, but is not readily available. Smallpox is a virus. It is highly contagious, transmits through the atmosphere very easily and has a high mortality rate.