Biological weapons can be either bacteria or a virus. There are two ways that they are obtained, by getting them from the outside soil or they are engineered in laboratories. The advantages of engineering a virus or bacteria is that you can control it to your needs. These weapons do not only have human as their targets, crops and animals are also targets . Every day more advanced biological weapons are being made that are deadlier.

http://en.wikipedia.org/wiki/Biological_warfare#Modern_times

How many different types of biological weapons are there?

What are the advantages of using biological weapons?