A poster has commented:
The cause of the poisoning is Po-210, see
Russian ex-spy murdered with alpha radiation with Po-210 in food in London
Incompetent medics failed to diagnose acute radiation poisoning from internal exposure to the heavy element polonium-210, the most deadly radioactive material on earth due to its short half-life of 140 days (plutonium-239 has a 24,400 years half life so each atom of that emits only one alpha particle per 35,100 years, which is a comparatively low dose rate - the average life is always 1.44 times the half-life with simply one-stage decay chain exponentially decaying radionuclides).
Notice Po-210 has a half-life of 140 days, and is a high-energy alpha emitter. Plutonium-239 for contrast has a half-life of 24,400 years so the specific activity of Po-210 (decays per second or Becquerels, per gram) is way higher. The shorter the half life, the more decays per second!Po-210 was used with beryllium as the neutron source (initiator) in the early 1945 nuclear weapons. Alpha particles hitting beryllium fission it, releasing neutrons. This was responsible for most of the deaths after the Windscale nuclear reactor fire in England in 1957. The pile was producing Po-210 for British nuclear bomb tests in Maralinga, but the government kept that secret, claiming that only iodine-131 had been released. (They didn’t want the Americans to know Britain was still using obsolete 1945 nuclear initiator technology!)