A Bioweapon is an item that has either been infected or has an infectious substance contained within. A biological weapon may be intended to kill, incapacitate, or seriously impede an individual as well as entire cities or places. Infectious substances used are Pathogens (Bacteria, viruses or other disease carrying agents.) Diseases most likely to be considered for use as biological weapons are contenders because of their lethality (if delivered efficiently), and robustness (making aerosol delivery feasible). One of the most well known and effective agent is Anthrax.
Once dropped, this Bioweapon will spread enough to infect the whole city.