The original report that he was poisoned with polonium mentioned how much they found on his body. Well, it has a half-life of about six months, so I backed it up to the day he was buried. There would have been so much Po210 around there that everyone at the funeral or who prepared the body would be dead, and probably anyone in their households too if they tracked it home.
Symptom of acute lead ingestion. Or any other number of things.