To spite its very tiny size, at any given time the blue-ringed octopus carries enough venom within its body to kill 26 adult human beings at a time.

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The venom works quickly, killing within minutes and there is no known cure for the poison. If medical care is given quickly enough to start artificial respiration and possible heart massage to keep a person breathing long enough for the poison to work itself out of a victims system, there is a chance to save the patient; however, given how quickly the venom works, time is of the essence.
The blue-ringed octopus carries two types of venom, one which is used for its prey when hunting; this particular venom poses little risks to humans. The second, deadlier venom, is the defensive mechanism to help protect the octopus from predators. Typically, this small and deadly creature only uses its defensive venom when provoked, therefore, humans are generally only attacked when they step on the small creature or begin harassing it trying to pick it up, etc.