Over the past few centuries, arsenic poisoning has been a particularly popular way to kill someone. It's odorless, tasteless, and builds up in the human body.
A large dose will kill someone in hours, while a steady, small dose will cause someone to become ill and appear to die from natural causes. The poison used to be extremely difficult to detect after death, until James Marsh developed a reliable test in 1832. Even after that, only the victims of suspicious deaths were tested - so many arsenic killers tallied up multiple victims before being caught.