If the terrorists can't manage to scare the hell out of people, their unwitting accomplices will. Terrorists rely on the fear and panic, and not any actual attacks to do the most damage.

Diseases which are transmitted primarily by contact with blood and other body fluids are transmitted that way because they can't survive long outside the body.

Diseases transmitted by air have to get into the air somehow. Thinking someone could just plop a mason jar down by the side of the road, and the infectious agent would just sort of float up into the air is ridiculous.

Dispersal methods are a huge limiting factor in biological warfare. They need an agent that will survive the dispersal, and will remain alive and present long enough to infect people, and not give itself away by killing animals and birds before it infects people.