How do Christian Scientists respond to the "poison that nobody knows is poisonous" argument?

Christian Scientists claim that disease is not caused by physical abnormality, but by a mental error. Skeptics sometimes use this argument against that: "Suppose somebody drinks a glass of water that contains a poison, but nobody knows that it is poisonous. If Christian Science is right, then that poison could not kill that person. If medical science is right, then it can kill that person. And it is obvious which is true.". How do Christian Scientists respond to that argument? Such things, although very rare, do happen in reality. Maria Curie died because of radiation which nobody back then knew was harmful. So did the Chinese emperor Qin Shi Huang die because of mercury poisoning which nobody back then knew was harmful.