The Romans, Greeks, Persians, basically all of the bigger civilizations around the Mediterranean area did this practice where they would take babies from their mothers within days of being born, and they would feed them small amounts of poison, but not enough to kill them.
Over the next 10 years or so, the children would continuously eat trace amounts of whatever poison was initially given to them, and after awhile the poison would build up in their systems, making them immune to not only that particular poison, but also any poisons of a similar nature.
A side-effect of this was that the children themselves would become poisonous, and any exposure to bodily fluids, or even a simple cut from one of their finger nails would be enough to kill a person. Eventually (usually around age 30 or so), the poison would toxify the bloodstream so much that the children would die from ingesting their own saliva.
The children of this practice, were called the Catevari, meaning "Those Who Are Cursed" in old Latin.