I'm killing off a character, but I need the killer to get away with it, and I wanted their death to be somewhat slow.

I initially used arsenic as the basis for my poison, but the illnesses I've found that it mimics are dysentery and cholera and those are both fairly quick killing. I've already had the character fall ill before and recover somewhat. Maybe that was a mistake?

I've read a bit about chronic arsenic poisoning and that it was hard to detect, but can't seem to find information on what illnesses it might have been mistaken for. Does anyone know of an illness/disease that this sort of slow gastrointestinal decline would be indicative of?

Thanks so much!!!

P.S. This is for a fantasy novel set in a world - time period - similar to the 1880's.