Grant,
His arsenic poisoning is actually something they discovered fairly recently. Is that Longwood House had several rooms with wallpaper that contained a dye called Scheele's green. It contained high amounts of arsenic, but was generally safe in dry conditions. Naturally an island is usually a little on the moist side. The paper therfore developed mold which if formed on the green pigment, released arsenic into the air, slowly poisoning anyone exposed to it for a long period of time, such as life imprisonment.
See, simple as that.
Talk to Ya Soon, Taylor