Polonium-210

a victim would never taste a lethal dose in food or drink...Another assassin advantage is that illness comes on gradually, making it hard to pinpoint the event. Yet another advantage is that polonium poisoning is so rare that it’s not part of a standard toxicology screen...And finally, it gets the job done. “Once absorbed,” notes the U.S. Regulatory Commission, “The alpha radiation can rapidly destroy major organs, DNA and the immune system.”

 Yasser Arafat and the Mysteries of Polonium-210