Clostridium botulinum is the name of a group of bacteria commonly found in soil. These rod-shaped organisms grow best in low oxygen conditions. The bacteria form spores which allow them to survive in a dormant state until exposed to conditions that can support their growth. Clostridium botulinum is the bacterium that produces botulin, the nerve toxin that causes botulism. Botulin is one of the most powerful toxins in the world; the lethal dose for a human has been estimated at 0.03 mg of dried toxin.