Pandemic biological warfare refers to the use of toxins and contagious agents such as viruses or bacteria, to cause the death or incapacitation of populations across wide regions such as continents or even worldwide.
For example, in 1346, the corpses of Mongol warriors who had been killed by plague were thrown over the walls of the besieged Crimean city of Kaffa, to infect the inhabitants inside. Some experts believe that this led to the emergence of the plague that caused the Black Death in Europe.
Similarly, the Native American population was exposed to many fatal diseases after contact with the Old World. One case of germ warfare that was documented involved British commander Jeffrey Amherst and Swiss-British officer Colonel Henry Bouquet who wrote to each other about the possibility of introducing blankets infected with smallpox to Indians as part of Pontiac's Rebellion, an event which occurred during the Siege of Fort Pitt in 1763 towards the end of the French and Indian War. It is still not known whether this particular event took place but after the war, at least 400,000 Native Americans are thought to have died from smallpox.
Between 1937 and 1945 during the Sino-Japanese War, a unit of the Imperial Japanese Army experimented on thousands of Chinese people, testing biological weapons on Chinese soldiers and civilians. The army used plague fleas, infected clothing and pathogens contained in bombs that were dropped to target various regions. Around 400,000 Chinese civilians are thought to have died from plague, cholera and anthrax as a result.
Other examples of infectious organisms that have either been used as a biological weapon or considered as a weapon include ebola, Marburg virus, tularemia, Rocky Mountain spotted fever, Q fever, brucellosis, typhus, Coccidioides mycosis, Psittacosis, Melioidosis, Glanders, machupo, Shigella, Rift Valley fever, yellow fever and Japanese B encephalitis.