There was a theory a while back that Napoleon died from arsenic poisoning due to the damp Mediterranean climate causing the decay of a green wallpaper using an arsenic based dye. It didn’t end up being entirely true but he may have been slowly poised by environmental arsenic throughout his life. Here is an article about it.

Arsenic is a naturally occurring element in the Earth’s crust as metallic crystals or combined with other elements. Being a heavy metal, arsenic bioaccumulates. That is, it builds up in the body over time, so slow poisoning is possible.