Why is the deadliest natural toxin known also injected into people on purpose?

Because what makes it deadly, stopping nerves from signalling muscles, is also what makes it useful in tiny, targeted amounts. Botulinum toxin, made by the bacterium Clostridium botulinum, is considered the deadliest known natural substance. It prevents nerve endings from releasing acetylcholine, the messenger that tells a muscle to contract, so the muscle goes limp. In botulism, that paralysis can reach the chest muscles and cause respiratory failure, and 5 to 10% of people who get the disease die. In strongly diluted form, injected into a single muscle, the same effect relaxes just that muscle.

The idea is surprisingly old. Botulism was first known as "sausage poisoning": in 1793, six people died in the German village of Wildbad after eating blood sausage, and the name comes from the Latin botulus, sausage. Between 1817 and 1822 the German physician Justinus Kerner described the disease in detail, and he became the first to suggest that the "sausage toxin" might treat illnesses caused by an overactive nervous system. Much later, the eye specialist Alan B. Scott and colleagues found that a few picograms injected into a monkey's eye muscle paralysed only that muscle, without side effects. Scott treated the first patients with misaligned eyes in 1977, and after US approval in 1989 the drug was sold as Botox. Today it is also used for chronic migraine and to reduce facial wrinkles.

In food, the trouble starts with spores. They are common in soil and water, very resistant, and produce toxin in low-oxygen conditions, which is why foodborne botulism typically comes from improperly prepared home-canned food or fermented dishes without enough salt or acidity. Boiling at sea level kills the active bacteria but not the spores, so commercial canning uses a pressurised "botulinum cook"; the toxin itself is destroyed by more than five minutes above 85 °C. Honey can carry the spores and has been linked to infant botulism, which is why it is not recommended for babies under one year old. Adverse effects from injections generally occur only with inappropriate cosmetic doses or the larger doses used for movement disorders.