A biological weapon or bio-agent was a weapon that operates by spreading a pathogen such as a virus or a bacterium to the intended targets. This weapon is generally either lethal or at least physiologically devastating to those afflicted. Genetic engineering was not infrequently used in the creation of such weapons. Nanotechnology was also known to be used on occasion. A sub-category, bio-chemical weapons, combined elements, usually artificially, of both biological and chemical weapons. One example of such weaponry was advanced pathogenic toxins, with another being synthetic prion-based agents.
Malcolm Reed told General Gosis a lie during the interrogation when he said he and Captain Jonathan Archer were genetically enhanced and their immune systems were resistent to chemical and biological weapons. (ENT: "The Communicator")
On Earth, the use of biological weapons was outlawed (in theory at least) by the Geneva Protocol of 1928 and 2155. (DIS: "Context Is for Kings")
In 2268, Leonard McCoy, while studying an infection on Omega IV, found that it resembled the bacteriological warfare experiments waged on Earth in the 1990s. (TOS: "The Omega Glory")
The Tarellian civilization was wiped out by biological weapons during a civil war, leaving a small number of plague-carrying survivors who escaped, typically infecting the populations of other planets after establishing contact. (TNG: "Haven")
Metagenic weapons were a biological weapon capable of destroying all DNA it encountered, as well as planetary ecosystems. In 2369, Jean-Luc Picard, Beverly Crusher, and Worf were sent to Celtris III by Starfleet Intelligence to locate and destroy this biological weapon the Cardassians were supposedly developing. (TNG: "Chain Of Command, Part I")
The Dominion at times used biological weapons to make examples of populations that resisted their rule, including the Teplans, afflicted with the blight, and the inhabitants of Boranis III. (DS9: "The Quickening", "Inter Arma Enim Silent Leges")
In 2373, upon initially seeing a Species 8472 bio-ship, Commander Chakotay speculated that it was either a space-dwelling organism or biological weapon. (VOY: "Scorpion")
In a holoprogram titled The Voyager Encounter, the Emergency Medical Android had examined the Kyrian genome and determined they would be vulnerable to any number of biological weapons. The Emergency Medical Android later established a datalink between his neural net and the phaser array of the warship Voyager to reconfigure the phaser beam to carry a bio-agent into the atmosphere of the Kyrian and Vaskan homeworld for the targeting of Kyrian cities. This attack caused 3,000 Kyrian deaths, however the fatality rate would be 300,000 in one hour as the bio-agent was still dispersing through the atmosphere. The death toll could've been much higher had there not already been an evacuation taking place which resulted in thousands of lives being saved. (VOY: "Living Witness")
In 2375, Species 8472 accused the USS Voyager of having created biological weapons and infecting their species. In addition they accused Voyager off allying with the Borg. (VOY: "In the Flesh")
The Brunali created a spaceborne virus that they then used to infect the Borg via children they deliberately allowed to be assimilated. Icheb was one such child, who (unknowingly) successfully transferred the virus to the Borg cube that assimilated him, killing all of the crew except for those Borg drones still in their maturation chambers. (VOY: "Collective", "Child's Play")