Of the many unpleasant ways to kill large numbers of people at once, biological weapons are perhaps the most nefarious.
The basic idea is to take a disease-causing bacteria (plague - yersina pestis - is a popular choice), grow it in large quantities, harvest it and then freeze-dry it. Once dried it can be milled into a fine powder, just the right size to be inhaled into the respiratory system and then it settles in the lungs. Once there, the disease can happily reproduce and incubate into a serious pulmonary strain which is extremely difficult to treat. Within two to three days, during which time the victim may become highly infectious, increasingly severe symptoms will escalate, varying according to the particular disease.
Biological weapons are silent, invisible, odourless and tasteless, and have a lethality rate (relative to weight and cost) higher than nuclear or chemical weapons. Hence they are often referred to as 'the Poor Man's Atomic Bomb'. A biological weapon dispersed over a population centre such as London or New York has the potential to cause thousands or even millions of deaths, dependent on the amount of agent dispersed, time of day, meteorological conditions, etc.