A biological war or bacteriological warfare was a type of war which made use of biological agents, such as viruses.
In 2268, Doctor Leonard McCoy compared an infection present on Omega IV to one developed on Earth in the 1990s during bacteriological warfare experiments. He found Humans' participation in such risky experiments to have been foolish. He later attributed the presence of the virus to a biological war sometime in Omega IV's past. Spock later suggested that the low population of the planet was either due to nuclear devastation or a bacteriological holocaust. (TOS: "The Omega Glory")