I think you have more to fear from the flu than from a terrorist attack. Some are contagious, some not. BW agents can attack us, our food animals, or our crops. They can be deadly, or just cause temporary effects. Thus, it is impossible to answer your question about symptoms because it would depend on which agent had been used.
Anthrax is one of the most likely agents to be used as a weapon.
It is a bacterium whose spores, when inhaled, cause fever and fatigue after 1 to 6 days. After 2-3 days, respiratory distress and chest pains begin. This is followed by shock and death within 24-36 hours. The disease can be treated effectively with antibiotics, but only if treatment is begun a day or less after exposure. There is an effective vaccine, but it is currently only available to the military and to people whose trades expose them to anthrax-such as tanners or others who may work with infected animal skins.