Biological warfare
Overview: A biological attack could conceivably result in large numbers of civilian casualties and cause severe disruption to economic and societal infrastructure.
A nation or group that can pose a credible threat of mass casualty has the ability to alter the terms under which other nations or groups interact with it. When indexed to weapon mass and cost of development and storage, biological weapons possess destructive potential and loss of life far in excess of nuclear, chemical or conventional weapons. Accordingly, biological agents are potentially useful as strategic deterrents, in addition to their utility as offensive weapons on the battlefield.
As a tactical weapon for military use, a significant problem with biological warfare is that it would take days to be effective, and therefore might not immediately stop an opposing force. Some biological agents (smallpox, pneumonic plague) have the capability of person-to-person transmission via aerosolized respiratory droplets. This feature can be undesirable, as the agent(s) may be transmitted by this mechanism to unintended populations, including neutral or even friendly forces. Worse still, such a weapon could "escape" the laboratory where it was developed, even if there was no intent to use it – for example by infecting a researcher who then transmits it to the outside world before realizing that they were infected. Several cases are known of researchers becoming infected and dying of Ebola, which they had been working with in the lab (though nobody else was infected in those cases) – while there is no evidence that their work was directed towards biological warfare, it demonstrates the potential for accidental infection even of careful researchers fully aware of the dangers. While containment of biological warfare is less of a concern for certain criminal or terrorist organizations, it remains a significant concern for the military and civilian populations of virtually all nations.

History: 

Antiquity and Middle Ages: Rudimentary forms of biological warfare have been practiced since antiquity. The earliest documented incident of the intention to use biological weapons is recorded in Hittite texts of 1500–1200 BCE, in which victims of an unknown plague (possibly tularemia) were driven into enemy lands, causing an epidemic.  The Assyrians poisoned enemy wells with the fungus ergot, though with unknown results. Scythian archers dipped their arrows and Roman soldiers their swords into excrements and cadavers – victims were commonly infected by tetanus as result. In 1346, the bodies of Mongol warriors of the Golden Horde who had died of plague were thrown over the walls of the besieged Crimean city of Kaffa. Specialists disagree about whether this operation was responsible for the spread of the Black Death into Europe, Near East and North Africa, resulting in the deaths of approximately 25 million Europeans.
Biological agents were extensively used in many parts of Africa from the sixteenth century AD, most of the time in the form of poisoned arrows, or powder spread on the war front as well as poisoning of horses and water supply of the enemy forces. In Borgu, there were specific mixtures to kill, hypnotize, make the enemy bold, and to act as an antidote against the poison of the enemy as well. The creation of biologicals was reserved for a specific and professional class of medicine-men.

18th to 19th century: During the French and Indian War, in June 1763 a group of Native Americans laid siege to British-held Fort Pitt. The commander of Fort Pitt, Simeon Ecuyer, ordered his men to take smallpox-infested blankets from the infirmary and give it to a Lenape delegation during the siege. A reported outbreak that began the spring before left as many as one hundred Native Americans dead in Ohio Country from 1763 to 1764. It is not clear whether the smallpox was a result of the Fort Pitt incident or the virus was already present among the Delaware people as outbreaks happened on their own every dozen or so years and the delegates were met again later and seemingly had not contracted smallpox. During the American Revolutionary War, Continental Army officer George Washington mentioned to the Continental Congress that he had heard a rumor from a sailor that his opponent during the Siege of Boston, General William Howe, had deliberately sent civilians out of the city in the hopes of spreading the ongoing smallpox epidemic to American lines; Washington, remaining unconvinced, wrote that he "could hardly give credit to" the claim. Washington had already inoculated his soldiers, diminishing the effect of the epidemic. Some historians have claimed that a detachment of the Corps of Royal Marines stationed in New South Wales, Australia, deliberately used smallpox there in 1789. Dr Seth Carus states: "Ultimately, we have a strong circumstantial case supporting the theory that someone deliberately introduced smallpox in the Aboriginal population."

World War I: By 1900 the germ theory and advances in bacteriology brought a new level of sophistication to the techniques for possible use of bio-agents in war. Biological sabotage in the form of anthrax and glanders was undertaken on behalf of the Imperial German government during World War I (1914–1918), with indifferent results. The Geneva Protocol of 1925 prohibited the first use of chemical and biological weapons against enemy nationals in international armed conflicts.

World War II: With the onset of World War II, the Ministry of Supply in the United Kingdom established a biological warfare program at Porton Down, headed by the microbiologist Paul Fildes. The research was championed by Winston Churchill and soon tularemia, anthrax, brucellosis, and botulism toxins had been effectively weaponized. In particular, Gruinard Island in Scotland, was contaminated with anthrax during a series of extensive tests for the next 56 years. Although the UK never offensively used the biological weapons it developed, its program was the first to successfully weaponize a variety of deadly pathogens and bring them into industrial production. Other nations, notably France and Japan, had begun their own biological weapons programs.
When the United States entered the war, Allied resources were pooled at the request of the British. The U.S. then established a large research program and industrial complex at Fort Detrick, Maryland, in 1942 under the direction of George W. Merck. The biological and chemical weapons developed during that period were tested at the Dugway Proving Grounds in Utah. Soon there were facilities for the mass production of anthrax spores, brucellosis, and botulism toxins, although the war was over before these weapons could be of much operational use.

The most notorious program of the period was run by the secret Imperial Japanese Army Unit 731 during the war, based at Pingfan in Manchuria and commanded by Lieutenant General Shirō Ishii. This biological warfare research unit conducted often fatal human experiments on prisoners, and produced biological weapons for combat use. Although the Japanese effort lacked the technological sophistication of the American or British programs, it far outstripped them in its widespread application and indiscriminate brutality. Biological weapons were used against Chinese soldiers and civilians in several military campaigns. In 1940, the Japanese Army Air Force bombed Ningbo with ceramic bombs full of fleas carrying the bubonic plague. Many of these operations were ineffective due to inefficient delivery systems, although up to 400,000 people may have died. During the Zhejiang-Jiangxi Campaign in 1942, around 1,700 Japanese troops died out of a total 10,000 Japanese soldiers who fell ill with disease when their own biological weapons attack rebounded on their own forces.
During the final months of World War II, Japan planned to use plague as a biological weapon against U.S. civilians in San Diego, California, during Operation Cherry Blossoms at Night. The plan was set to launch on 22 September 1945, but it was not executed because of Japan's surrender on 15 August 1945.

Cold War: In Britain, the 1950s saw the weaponization of plague, brucellosis, tularemia and later equine encephalomyelitis and vaccinia viruses, but the programme was unilaterally cancelled in 1956. The United States Army Biological Warfare Laboratories weaponized anthrax, tularemia, brucellosis, Q-fever and others.
In 1969, US President Richard Nixon decided to unilaterally terminate the offensive biological weapons program of the US, allowing only scientific research for defensive measures. This decision increased the momentum of the negotiations for a ban on biological warfare, which took place from 1969 to 1972 in the United Nation's Conference of the Committee on Disarmament in Geneva. These negotiations resulted in the Biological Weapons Convention, which was opened for signature on 10 April 1972 and entered into force on 26 March 1975 after its ratification by 22 states.
Despite being a party and depositary to the BWC, the Soviet Union continued and expanded its massive offensive biological weapons program, under the leadership of the allegedly civilian institution Biopreparat. The Soviet Union attracted international suspicion after the 1979 Sverdlovsk anthrax leak killed approximately 65 to 100 people.

1948 Arab–Israeli War: According to historians Benny Morris and Benjamin Kedar, Israel conducted a biological warfare operation codenamed "Cast Thy Bread" during the 1948 Arab–Israeli War. The Haganah initially used typhoid bacteria to contaminate water wells in newly cleared Arab villages to prevent the population including militiamen from returning. Later, the biological warfare campaign expanded to include Jewish settlements that were in imminent danger of being captured by Arab troops and inhabited Arab towns not slated for capture.