Biological weapons are designed to cause numerous deaths or serious wounds using living or dead bodies or the toxic products those bodies produce. Biological weapons are very cheap and easy to manufacture - anyone with a degree in biology could easily produce a deadly disease.
Biological attacks are much easier to execute than chemical attacks and their consequences are more difficult to predict. An attack using biological agents can be triggered by, for example, placing the agent in a water supply, releasing the agent at crucial points in the food chain (like a meat processing plant), releasing contaminated rodents or releasing the agent in the air to be inhaled by the general population.
Viral agents require a living cell to function and cause diseases like smallpox and yellow fever. While we can be vaccinated against viral diseases, once infected, there is no antibiotic treatment possible.
Bacterial agents are single-cell organisms that are capable of growing and reproducing themselves. They cause such epidemics as anthrax, cholera, plague and tularemia. Bacterial diseases can be treated with antibiotics provided they are detected early enough.
Some of the most commonly discussed biological agents are smallpox, plague and anthrax. Smallpox and plague are the more dangerous because they spread easily from one person to another. Anthrax doesn't spread from person to person meaning that its consequences would be confined to the immediate vicinity of an attack.