Botulinum toxin is one of the most lethal toxic substances on Earth, but ironically has a dual-use as a drug for migraines and neuromuscular disorders. Its LD50 (the dose at which half of those exposed can be predicted to die) is about a nanogram per kilogram of body mass, making the typical lethal dose for adults in the 50-100 nanogram range. For comparison, the mass of an average human cell is one nanogram. Therapeutic doses of botulinum are significantly below even this tiny mass. When used commercially, botulinum toxin goes by the brand names Botox® and Dysport®.
The botulinum toxin is produced by the bacterium Clostridium botulinum, which can be found in soil around the planet and is cultured easily. The main reason why millions don't day from exposure to it daily is because it degrades rapidly on contact with air. This is also why it is seen as an inferior agent for chemical warfare. However, if delivered robotically by insect-sized drones, it could be a superior chemical weapon.
Botulinum toxin is used therapeutically for cosmetic purposes because of its relaxant properties. In extremely tiny amounts, it blocks the release of acetylcholine at the neuromuscular junction, counteracting the effects of aging, which cause facial muscles to tense up and produce lines. Cosmetic administrations of botulinum are relatively short-term endeavors, lasting 3-6 months but sometimes as short a time as a few weeks. Slight side effects such as eyelid drooping or uneven smile are possible risks of the therapy. Despite this, it is thought by some that John Kerry used botulinum therapy during the 2004 Presidential elections in an effort to look younger.
It is often cited that only a few hundred grams of botulinum toxin would be enough to kill every human being on Earth. Exposure is a risk in certain types of canned food with holes that let the spores get in. Although the toxin rapidly degenerates upon exposure to air, the spores do not, and can even withstand extended boiling. Fortunately, few cases of such poisoning have been reported. The toxin was named after the Latin word for sausage, botulus, because one of the first places it was discovered was on rotting sausage.