Biological warfare
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. There was also plans to expand the biological warfare campaign into other Arab states including Egypt, Lebanon and Syria, but they were not carried out.

International law: International restrictions on biological warfare began with the 1925 Geneva Protocol, which prohibits the use but not the possession or development of biological and chemical weapons in international armed conflicts. Upon ratification of the Geneva Protocol, several countries made reservations regarding its applicability and use in retaliation. Due to these reservations, it was in practice a "no-first-use" agreement only.
The 1972 Biological Weapons Convention (BWC) supplements the Geneva Protocol by prohibiting the development, production, acquisition, transfer, stockpiling and use of biological weapons. Having entered into force on 26 March 1975, the BWC was the first multilateral disarmament treaty to ban the production of an entire category of weapons of mass destruction. As of March 2021, 183 states have become party to the treaty. The BWC is considered to have established a strong global norm against biological weapons, which is reflected in the treaty's preamble, stating that the use of biological weapons would be "repugnant to the conscience of mankind". The BWC's effectiveness has been limited due to insufficient institutional support and the absence of any formal verification regime to monitor compliance.
In 1985, the Australia Group was established, a multilateral export control regime of 43 countries aiming to prevent the proliferation of chemical and biological weapons.
In 2004, the United Nations Security Council passed Resolution 1540, which obligates all UN Member States to develop and enforce appropriate legal and regulatory measures against the proliferation of chemical, biological, radiological, and nuclear weapons and their means of delivery, in particular, to prevent the spread of weapons of mass destruction to non-state actors.

Bioterrorism: Biological weapons are difficult to detect, economical and easy to use, making them appealing to terrorists. The cost of a biological weapon is estimated to be about 0.05 percent the cost of a conventional weapon in order to produce similar numbers of mass casualties per kilometer square. Moreover, their production is very easy as common technology can be used to produce biological warfare agents, like that used in production of vaccines, foods, spray devices, beverages and antibiotics. A major factor in biological warfare that attracts terrorists is that they can easily escape before the government agencies or secret agencies have even started their investigation. This is because the potential organism has an incubation period of 3 to 7 days, after which the results begin to appear, thereby giving terrorists a lead.
A technique called Clustered, Regularly Interspaced, Short Palindromic Repeat (CRISPR-Cas9) is now so cheap and widely available that scientists fear that amateurs will start experimenting with them. In this technique, a DNA sequence is cut off and replaced with a new sequence, e.g. one that codes for a particular protein, with the intent of modifying an organism's traits. Concerns have emerged regarding do-it-yourself biology research organizations due to their associated risk that a rogue amateur DIY researcher could attempt to develop dangerous bioweapons using genome editing technology.
In 2002, when CNN went through Al-Qaeda's (AQ's) experiments with crude poisons, they found out that AQ had begun planning ricin and cyanide attacks with the help of a loose association of terrorist cells. The associates had infiltrated many countries like Turkey, Italy, Spain, France and others. In 2015, to combat the threat of bioterrorism, a National Blueprint for Biodefense was issued by the Blue-Ribbon Study Panel on Biodefense. Also, 233 potential exposures of select biological agents outside of the primary barriers of the biocontainment in the US were described by the annual report of the Federal Select Agent Program.
Though a verification system can reduce bioterrorism, an employee, or a lone terrorist having adequate knowledge of a bio-technology company's facilities, can cause potential danger by utilizing, without proper oversight and supervision, that company's resources. Moreover, it has been found that about 95% of accidents that have occurred due to low security have been done by employees or those who had a security clearance.

Entomology: Entomological warfare (EW) is a type of biological warfare that uses insects to attack the enemy. The concept has existed for centuries and research and development have continued into the modern era. EW has been used in battle by Japan and several other nations have developed and been accused of using an entomological warfare program. EW may employ insects in a direct attack or as vectors to deliver a biological agent, such as plague. Essentially, EW exists in three varieties. One type of EW involves infecting insects with a pathogen and then dispersing the insects over target areas. The insects then act as a vector, infecting any person or animal they might bite. Another type of EW is a direct insect attack against crops; the insect may not be infected with any pathogen but instead represents a threat to agriculture. The final method uses uninfected insects, such as bees or wasps, to directly attack the enemy.

Genetics: Theoretically, novel approaches in biotechnology, such as synthetic biology could be used in the future to design novel types of biological warfare agents.

Would demonstrate how to render a vaccine ineffective;
Would confer resistance to therapeutically useful antibiotics or antiviral agents;
Would enhance the virulence of a pathogen or render a nonpathogen virulent;
Would increase the transmissibility of a pathogen;
Would alter the host range of a pathogen;
Would enable the evasion of diagnostic/detection tools;
Would enable the weaponization of a biological agent or toxin.
Most of the biosecurity concerns in synthetic biology are focused on the role of DNA synthesis and the risk of producing genetic material of lethal viruses (e.g. 1918 Spanish flu, polio) in the lab. Recently, the CRISPR/Cas system has emerged as a promising technique for gene editing. It was hailed by The Washington Post as "the most important innovation in the synthetic biology space in nearly 30 years." While other methods take months or years to edit gene sequences, CRISPR speeds that time up to weeks. Due to its ease of use and accessibility, it has raised a number of ethical concerns, especially surrounding its use in the biohacking space.

By target: 

Anti-personnel: Ideal characteristics of a biological agent to be used as a weapon against humans are high infectivity, high virulence, non-availability of vaccines and availability of an effective and efficient delivery system. Stability of the weaponized agent (the ability of the agent to retain its infectivity and virulence after a prolonged period of storage) may also be desirable, particularly for military applications, and the ease of creating one is often considered.  Control of the spread of the agent may be another desired characteristic.
The primary difficulty is not the production of the biological agent, as many biological agents used in weapons can be manufactured relatively quickly, cheaply and easily. Rather, it is the weaponization, storage, and delivery in an effective vehicle to a vulnerable target that pose significant problems.
For example, Bacillus anthracis is considered an effective agent for several reasons. First, it forms hardy spores, perfect for dispersal aerosols. Second, this organism is not considered transmissible from person to person, and thus rarely if ever causes secondary infections. A pulmonary anthrax infection starts with ordinary influenza-like symptoms and progresses to a lethal hemorrhagic mediastinitis within 3–7 days, with a fatality rate that is 90% or higher in untreated patients. Finally, friendly personnel and civilians can be protected with suitable antibiotics.
Agents considered for weaponization, or known to be weaponized, include bacteria such as Bacillus anthracis, Brucella spp., Burkholderia mallei, Burkholderia pseudomallei, Chlamydophila psittaci, Coxiella burnetii, Francisella tularensis, some of the Rickettsiaceae (especially Rickettsia prowazekii and Rickettsia rickettsii), Shigella spp., Vibrio cholerae, and Yersinia pestis. Many viral agents have been studied and/or weaponized, including some of the Bunyaviridae (especially Rift Valley fever virus), Ebolavirus, many of the Flaviviridae (especially Japanese encephalitis virus), Machupo virus, Coronaviruses, Marburg virus, Variola virus, and yellow fever virus. Fungal agents that have been studied include Coccidioides spp.
Toxins that can be used as weapons include ricin, staphylococcal enterotoxin B, botulinum toxin, saxitoxin, and many mycotoxins.