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?

Biological weapons have been used since ancient times. In the 16th century the Assyrians poisened the enemy wells with ergo, which was a kind of fungus that made the enemy delusional. Also during medieval times victims of the plague were flung over the castle walls with catapults to infect the people in the castle. Now there are more modern biological weapons in use. But since there was a treaty made that during warfare biological weapons werent being used, they are mostly used by terrorists.

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

In what other cultures were biological weapons used?

What other methods were used to transmit the disease?