Like Randy said, the distinction between natural and manmade poisons is not one of degree, but of chemistry

> From a public health point of view, in addition to the toxic industrial chemicals found in insecticides and pesticides, poisoning by some of the natural neurotoxins continues to be an important health hazard in man, particularly in developing tropical countries. For most of the toxins, there are no antidotes; assisted ventilation in case of respiratory paralysis is the cornerstone of treatment. The fatality rates are high, and death may occur within a few hours before the victim can reach a medical facility, emphasizing the importance of prevention of exposure. 

> A variety of natural toxins of animal, plant, and bacterial origin are capable of causing disorders of neuromuscular transmission. Animal toxins include venomous snakes and arthropods, venoms of certain marine creatures, skin secretions of dart-poison frogs, and poisonous fish, shellfish, and crabs. There are plant poisons such as curare, and bacterial poisons such as botulinum toxin. These act at single or multiple sites of the neuromuscular apparatus interfering with voltage-gated ion channels, acetylcholine release, depolarization of the postsynaptic membrane, or generation and spread of the muscle action potential. The specific actions of these toxins are being widely exploited in the study of neuromuscular physiology and pathology. Some toxins have proved to be valuable pharmaceutical agents. 

Senanayake, N., & Román, G. C. (1992). Disorders of neuromuscular transmission due to natural environmental toxins. Journal of the neurological sciences, 107(1), 1-13.

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