The most poisonous substances known: What makes some bacterial toxins so dangerous?

We are used to thinking of proteins as beneficial, so it is surprising to realize that the most toxic substances known to man are also protein molecules. These are the bacterial exotoxins, proteins secreted by pathogenic bacteria. Toxicity is measured by the median lethal dose (LD50). An LD50 value is defined as the mass of toxin per kg of body weight required to wipe out half of an animal population. Whereas classic poisons, such as potassium cyanide or arsenic trioxide, have LD50 values in the range 5–15 mg/ kg, the causative agent of botulism, botulinum toxin, has an LD50 in the range 1–3 ng/kg, a million times more toxic!