Biological units are StarCraft units that consist primarily of non-mechanical components.
Biological units are vulnerable to a wide array of effects, such as a Science Vessel's Irradiate ability and the Queen's Spawn Broodling ability. However, they are immune to effects such as the Ghost's Lockdown ability.
They include all Zerg units, Terran's SCV(also counted as mechanical), Marine, Firebat, Ghost and Medic, and the Protoss's Zealot, High Templar and Dark Templar.