...Small pox is perhaps the scariest agent for a biological weapons because it is the deadliest of all contagious human agents: once it spreads and people get infected they could spread to other people and they pass it on to other people and so on. The “hottest” strains kill as many as 50 percent of the people they infected. Some scenarios of a small pox attack have millions dead.
Smallpox can easily be sprayed into a crowded place. Victims wouldn't begin to display symptoms until 10 to 14 days after the attack. People who came in contact with them who have a good chance of contacting the disease. In the event of a terrorist attack, the disease could quickly overwhelm the health care system...